The difficulty is that Tibet is isolated by mountains to the west and south where Islam would enter. The Himalayas make a formidable barrier. It is extremely difficult for Muslims to invade Tibet. It is really only accessible through the interior of China, not through India or Afghanistan.
The same problem arises for Islam to penetrate peacefully through trade like it did on the East African Coast or the East Indies. There is also another problem with that, since a major reason Islam took hold is that 1) Islam brought a superior culture that brought law, literacy, and other cultural artifacts to some of those areas, and 2) a common religion enabled more uniform trading practices, established trust, and improved economic exchange. In SE Asia, Islam spread mainly through the native masses converting who were not Hindu or only superficially so. For the most part, only the elites were heavily Hinduized and influenced by the greater culture of India. The masses did not have access to that, but they could have access to the cultural treasures of Islam by converting. In those areas where Hinduism had become part of the popular religion, Islam had much more trouble spreading.
Tibet is not in a similar situation since they are already heavily influenced by Chinese culture and religion. Islam offers far fewer benefits to the Tibetans compared to what they already have. Compared to the costs of conversion (popular unrest, elite dissatisfaction), converting probably has a negative cost.
Islam is simply not established in the region early enough to provide a real initial competitor to Buddhism. Buddhism was already in place right before the great Islamic conquests in Central Asia. Although the culture as a whole did not become Buddhist until a century or more later, Tibet was dealing with Tang Dynasty China which was heavily influenced Buddhism. It is only a matter of time before some Tibetan Emperor converts and patronizes Buddhism.
For Islam to insert itself, you need China to become more much Muslim, which again is not possible in the early area. Islam has been a significant religion in China, although generally limited to its western parts (that has access to Central Asia) and the ports of Southern China (starting from the Song Dynasty) although never at the governing level. You would need to significantly increase the level of Islam in China before you can increase it in Tibet which means Tibet (and China) would have to convert long after both cultures are set in their ways. It could happen, but extremely unlikely.
Also, I would not overstate the amount of pacifism in Buddhism. The idea that Buddhists are somehow more pacifist than other religions is not really true. Christianity has an equally strong claim to pacifism, and look how that turns out in reality. Throughout history, there have been many examples of armed Buddhist monks fighting, of Buddhist politicians conquering, etc. Affairs of state generally take precedence over philosophical considerations. The idea that Buddhism means adherents won't fight is generally a result of people being ignorant with the regional history.