That's a great question! I hope you don't mind if I give a long answer? I apologize in advance to anyone who knows a lot about this: I'm going to go with names spelled as I remember them from the Spanish Chroniclers as much as possible, but I imagine I'm going to slip into an alternative from time to time as I am working from memory rather than having any one book open in front of me.

First off, it's important to understand the events leading up to Manco Inca's rebellion. At about the time of his birth, a civil war swept through Tahuantinsuyu --the Four Quarters of the Earth, as the Inca called their empire-- as the area roughly equivalent to modern Ecuador attempted to secede from the Inca Empire. The Emperor Huayna Capac (Manco's father) took his adult sons and spent the rest of his life in the north fighting a long and bloody war of reconquest. His eventual success, unfortunately, was rendered moot when a wave of European epidemics struck the empire before anyone that far south ever saw a Spaniard. By some estimates, one in three fell sick and one in four died, including Huayna Capac and his heir Ninan Cuyuchi.

A veteran army with battle-tested generals who had been living for many years in the north rallied to one of the eldest surviving princes, Atauhuallpa, who quickly drafted additional troops from the recently vanquished foe to swell his numbers. Meanwhile, the capital of Cuzco had been governed in Huayna Capac's absence by another prince named Huascar, and he promptly had himself named Emperor. A war between the brothers broke out, but the result was almost a foregone conclusion: Atauhuallpa had by far the better army and better commanders, and eventually his forces --although notably without him being present-- conquered Cuzco and captured Huascar and all the younger princes who had remained behind in the capital to grow up, including Manco Inca who was at that time in his late teens and considered an adult.

Atauhuallpa was making his way to Cuzco for the Inca equivalent of an official Imperial coronation when Pizarro and his small band of conquistadors landed. Atauhuallpa was intrigued by these strangers, and he allowed them to make their way inland and meet him at a famous hot springs. In what has to be one of the most daring moments in human history, less than 200 conquistadors captured Atauhuallpa alive and unharmed while he was in the midst of several thousand of his unarmed retainers and as an army of tens of thousands camped nearby without losing a single man. They then proceeded to ransom Atauhuallpa for an enormous sum of gold and silver. Meanwhile, Atauhuallpa almost certainly ordered Huascar's murder to deny Pizarro the opportunity to mediate between the brothers and choose the next emperor.

With the ransom almost paid in full, the conquistadors were growing increasingly nervous that their luck would run out. Sending away the men most likely to object, Pizarro set up a kangaroo court and had Atauhuallpa executed. He then picked one of Atauhuallpa's brothers to be his new puppet emperor, but that non-entity was quickly poisoned. This is where Manco finally comes in.

With Huascar dead and Atauhuallpa at first confined and then also dead, the northern army garrisoning Cuzco no longer had orders to hold the young princes who had been on the wrong side of the war between the brothers, and Manco Inca was the winner of the race to declare his allegiance to Pizarro in exchange for some small measure of power. He escorted the conquistadors to Cuzco, and he then watched as they pillaged the capital of gold, silver, food, and women. We have a lengthy list of insults Manco Inca bore as the Spaniards' puppet, and one day he had enough.

This brings us to your question: Why did he not attempt to split apart the alliance between the Native Auxiliaries and the Spanish Conquistadors? Well, to an extent he did. He himself was one of their auxiliaries, after all, and every one of the men who answered his summons through the broken Imperial postal system was also standing in defiance of the new Spanish overlords. The trouble is, there just was not nearly enough of them. Now that we have the background, we can get into why that is.

First, let us remember that the vast majority of the veteran fighting men were in the north throughout Manco Inca's life, first on both sides of the war of reconquest under Huayna Capac, and then under the command of Atauhuallpa, who had beaten the Cuzco-based Huascar to whom Manco Inca owed his loyalty. None of those men knew Manco Inca: He was a baby when the ones loyal to his father marched away, and he was a political non-entity on the wrong side of a civil war when they marched back. They owed him no allegiance. Besides, they were scattered under various generals after Atauhuallpa's execution, and each of those armies were running for their lives from both Spanish-led forces and the people they had been persecuting through two wars for almost two decades. The north was so lost to Manco that at one point the Chacapoyas --a powerful and militant tribe famed for providing soldiers to the Empire-- burned one of his messengers at the stake in the style of the Spaniards saying, "Because the Bearded Ones are clearly never going to return to Castile but will always be here, we are all going to have become Christians. And since you Inca have always betrayed us, you are going to die."

Giving up the northern quarter as a lost cause to the Inca, then, let's look at the southern quarter. Here we have better prospects, as the Spaniards had not done much in the large southern quarter of the Empire as of the time of Manco's rebellion. We even see some small hints of unrecorded diplomacy, as the Inca High Priest during the time of Manco's rebellion had the improbable name of Vilauma, which means Blood Drinker in one of the languages from the southern quarter of the Empire suggesting the position had been given to someone outside the Children of the Sun. Unfortunately, Manco did not have the time or the resources or the means of communication or the authority over the local administrators to demand a proper levy for most of the southern tribes' available troops, and an interesting argument has been made that the best men available --the warlike Colla and Lupaca, for instance-- had long since seen their male population sent north into one of the earlier conflicts.

With the north and south a mixed bag of bad news, we can turn to the western quarter. Here we see a lot of action, most notably in the siege of Lima. Unfortunately, the people of the coastal desert --the Hot Lands as the mountain men called them-- had always had a difficult relationship with their Inca overlords. This relatively untouched population was not eager to rise up on the Inca's behalf, and those who did were willing to be discouraged when the Spaniards did not collapse immediately.

And so we turn to the eastern quarter of Tahuantinusu to find some good news for Manco: The few tribes of the Amazon rain forest who had ever bowed to the Children of the Sun did not abandon their masters when the world above in the high mountains started learning Spanish. When Manco's rebellion was defeated, he retreated into the jungles and set up a rump state there that survived until the time of his grandchildren.

For all that, I suppose your original question was directed at the men actively serving the Spaniards. Why did Manco not try to win them back? I would say it is a combination of everything I have already said, with one additional factor: The people who joined the Spaniards had been on either side of a series of wars in which they had no personal stake for so long that they only answered to their own immediate commanders in exchange for food, women, and a share of the booty. Alternatively, the non-veterans had been so brutalized by both sides of those civil wars that they viewed the Spaniards as at the very least no worse than the Inca, and quite possibly their salvation from the Inca. Manco Inca was a young man who almost no one knew until his was a Spanish puppet: Even assuming he could talk to the warlords who had joined the Spaniards, what could he possibly offer them that was better than what they already had?

He did his best, but it was not enough. I cannot imagine a scenario where Manco Inca could have led a successful rebellion against the Spaniards. The worst of the damage was done long before the Spaniards captured Atauhuallpa, and by the time Manco had any power at all, no one knew him or owed him anything, he had nothing to offer, and there was decades of bad blood against the Inca motivating people to pull the whole empire apart out of spite if nothing else.

Now, having written all of that, I really do need to cite some sources I suppose. I hope anyone reading this far will not mind if I mention I've actually written a historical novel about all this that has some decent reviews and seems to have found a following with people travelling in South America. To put that book together, I've read all of the primary sources that have been translated into English, as well as a lot of wonderful secondary sources. If you want to check my work against the primary sources, I believe it's worth mentioning that each one is based on interviews conducted many years after the events I have described with old men who were minor players on opposite sides of civil wars during the days of Huayna Capac and Atauhuallpa. Manco Inca took most of what was left of the Inca nobility with him into exile, so there are some clear conflicts based on understandable biases from one Spanish chronicler to the next. A few secondary sources who have collated these different accounts logically would be Hemming's The Conquest of the Inca, Prescott's History of the Conquest of Peru, and D'Altroy's The Incas.

EDIT: Typos.