Not through lack of trying.

But we do have some theories about this. For reference, my sources for this will be primarily the pages dealing with this in 'Genghis Khan: The Man Who Conquered the World' by Frank McLynn, as I actually have that to hand. However, I'd be willing to dig through his citations for the primary work if there's any major contentions about my answer. With that out of the way, let's get started.

In the aftermath of the destruction of Transoxiana by Genghis's forces, primarily between 1219 and 1221, it did seem as if the Mongols at the peak of their military efficiency might invade the Sultanate of Delhi. Specifically, to pursue the Khwarezmian Shah's heir, Jalal al-Din. However, with Jalal's forces broken and the Mongol tumens under Genghis worn out after the clashes, they turned away. Some Delhi commentators later claimed that this was out of fear of Indian cavalry, despite the fact that such claims were made about the Jin, Khwarezmian, and later Hungarian cavalry.

McLynn, instead, notes

In the first place, the summer heat in India was almost unberable. Whereas temperatures in Lahore and Delhi were between 60 and 65 degrees in winter, this rose to over 90 degrees in summer.

Furthermore, the problem of forage for the Mongolian armies was rapidly becoming egregious; and the actual supply of horseflesh an even more pressing concern. The wastage of mounts increased dramatically once the Mongols entered inhospitable terrain such as the forested area of Northern India and the mountains of Afghanistan. Even the sturdy, resistant steppe horse of Mongolia was finding the constant campaigning hard going. An invasion of Delhi without a surplus of remounts was a daunting prospect even for the Mongolian war machine at it's fighting peak.

Lastly, there were outside influences. The Xia Xia had repudiated their treaty to aid Muqali in Jin China, and Genghis desired to break them. He saw them as far more pressing matter, for he fundamentally associated Mongolian prestige with their ability to maintain their conquests. Add into this a growing 'sick-list' in his army, and we can clearly see some of the logic behind Genghis turning away from Delhi.

However, we must also ask ourselves why a later invasion by the Ilkhanate and Chagatai Khanate did not succeed in the 1290s. In essence, these armies no longer possessed the same advantages that their forefathers had, but their opponents had also not been idle. Not only were they able to fight the Mongolian armies in passes and at river crossings. Furthermore, many of the 'Mongolian' forces had actually returned to Mongolia; the vast majority of these armies were from Central Eurasian tribes such as the Cumans, but also from the inhabitants of Transoxiana and Persia. They utilised larger proportions of infantry in a central role, and were no longer backed up by the nomadic pastoral system of supply that Genghis's armies had undertaken. Culturally and militarily, the Chagatai Khanate and the Ilkhanate were vastly different from the tumens of their forefathers.

No military exists in a vacuum, and so the rulers of Delhi, Jalauddin and his successor Alauddin, had studied the armies of their neighbours closely. Their correct conclusions about the lack of discipline amongst the conscripted levies as opposed to the more martial nomadic horseman led to decisive victories over Mongolian armies.

So, there's my answer. I'm not sure the Mongols under Genghis lacked the ability to conquer India, but I do side with that view. However, the decision to turn away from Delhi was driven primarily by logistical and political issues. Later invasions were unable to conquer India because the Sultanate of Delhi had developed effective counter-measures to the military style of the surround Khanates.

I do hope this helped!