It seems like a pretty important event: Extremists take over the holiest site in Islam because they are angry with an "un-Islamic" Saudi government. But most history of Islamic terrorist studies I've read don't focus on it and turn instead to the Soviet invasion of Afghanistan.
Why is this? Was the siege unimportant in the history of Islamic terrorism?
Because it's sort of weird.
The group that did the attack was sort of a dead end group, it did not leave a lasting ideology or directly inspire future groups (as it was stupid levels of heretical to the wide spectrum of Shia and Sunni).
Where it does tie in is the concessions made to the ultra conservative Saudi clergy to gain their support for retaking the Grand Mosque. This led to a reversal of many of the more liberal trends going on in Saudi Arabia and fostered the Wahabbist interpretation of Islam that most Sunni terrorist groups ascribe to.
It'd likely get more attention of the group hadn't declared someone in their group to be the Mahdi and just been strictly ultra conservative.
I would disagree that it doesn't get attention, but I would agree that it doesn't get the focus. 1979 was a watershed year in modern history based solely on events that occurred in the greater Middle East. January '79, Iranian revolution. March '79, Egypt-Israel Peace. July '79, Saddam Hussein takes power in Iraq. November '79, Grand Mosque Seizure. December '79, The Soviets Invade Afghanistan.
On the merits I think you can entirely justify the case that the seizure of the grand mosque was, in fact, the least important of these events, but to say that it's not included in the history of terrorism I don't think is correct. It's included, for example, in both The Looming Tower and Ghost Wars which are basically the best histories to date of the prelude to 9/11 and the GWOT.
Most of the discussion about the Grand Mosque seizure that I've read, however, focuses on Saudi Arabia's reaction after the attack. That it basically reversed course from a period of reform and instead focused on exporting its religious ideology and limiting domestic reforms. That, in turn, had a direct bearing on the Saudi approach to the Afghan Jihad and to its approach to men like Osama Bin Laden.
The group that seized the mosque itself was also highly unorthodox, and were basically Mahdists. Insofar as most subsequent studies of terrorism have focused on Salafist Jihadists, or else on Shia terrorist groups, the group that seized the Grand Mosque was basically sui generis, so while they're certainly interesting they didn't leave any lasting doctrines or ideology. Jihadists today will still refer to "Sheikh Osama' whereas Juhayman al-Otaibi is basically irrelevant in modern jihadist discourse.