Your surprise in their response may be due to a misunderstanding of the actual size of the order, so let's take a look at the facts.

During the high point of the Kingdom of Jerusalem, under which the Order of the Temple flourished, they grew quickly in military and financial strength. This was through the award of gifts of land and income - they were attractive to the ruling dynasty and its nobility because the knights' upkeep was covered by the Order and not the crown. However, the Templars strength was in the control of strategic places, not open warfare. Garrisons of the Order at places like Gaza could have an impact far greater than their size would suggest, in terms of raiding trade routes and disrupting enemy lines of communication. Still, the Templars would only ever have perhaps 250-300 field-ready knights at most, with a support network of squires and footmen numbering twice that. (Smail, 1995)

Does that make them a significant military? Well, the field armies of the Crusader states before 1187 were larger, as were most crusader armies from the West. Consider the Second Crusade of Conrad III and Louis VII, which comprised the best part of 3000 knights at the start of the march in Anatolia. (Runciman, 1951) Compared to the army of a leading European prince in his home lands, such as the King of France, the Templars at their zenith could not have entered into a military conflict.

Let us fast forward to their arrest in 1307. The Templars are no longer at the pinnacle of their strength. Why? Firstly, the loss of Acre in 1291 has effectively ended the period of pro-active crusading in Palestine and Syria proper. This severed a source of income for the Order, which had been a huge holder of land in the region (much to the annoyance of writers like William of Tyre). During the Third Crusade grandmaster Robert de Sable (yes, the Assassins Creed guy) had purchased Cyprus from Richard I of England, but they no longer held that either. While they were still wealthy from their business and financial interests in Europe, they were no long the transcontinental force they had been.

In addition, the legitimacy of the Order was papal, as the grandmaster was absolved of obedience from any Earthly authority save that of the pope. When Clement V, who later transferred the papal court to Avignon, ordered the Templars to be suppressed, they were effectively out of options.

So let's sum up. They were powerful in military terms, but in a relative way. Their strength was based on a very particular and context-sensitive style of combat which was effective in the East. Their numbers, even at their height, made them no match for the princes of Europe. They were also rich, but had seen their Middle Eastern estates eroded for decades and then finally extinguished almost entirely. In the West, they could operate as long as the patronage from Rome endured - without it they did not have the physical resources to resist a direct attack.

This was just a super quick couple of thoughts, but if you want to know more check out The New Knighthood by Barber, which is all about the Templars and will give you a great insight I think!

A couple of reasons can be cited. First, the major knightly orders (Templars, Teutonic Knights and Knights Hospitaller) were divided into two different organisations: the armed part which was for the most part stationed in the Kingdom of Jerusalem and the "Homefront". The latter managed the holdings of the order, trained and equipped recruits, but did not posess large military forces. Also, the holdings were spread all over Europe, so even the forces that were avaiable were not concentrated in any one place.

Second, the arrests were supported both by the King of France as well as the Pope. Resistance would have meant going into open rebellion against the state and the church. This was not really a possibility, as the order was legally part of the church.

Third, the arrest was carried out as a surprise attack. The grand master was arrested right at the start in Paris. The King of France had planned the arrests in advance (signing the warrant on September 14. and having them distributed in France with the order to open the letters in October), and was able to seize most of the order's assets and members. The Templars did keep assets in other places like England and Scotland for some years, but their main base was France.

In short, there was never a chance for organised resistance and it is unlikely that the order would have even tried to resist if it had been warned in advance.