Why no Islamist attacks on the Soviet Union?

That requires you to be in the city. The USSR's oppressed minorities (and Russia's oppressed minorities today) were not in the main cities, but in peripheral areas.

Which did not stop a couple nasty attacks such as Dubrovka Theater to be carried out.

I gather that there is plenty of Muslims of Central Asian and Caucasian origin in Moscow.
Furthermore, there are some fairly major cities in Muslim majority areas in Russia today (Kazan is probably the largest) and even more so in the Soviet Union.
However, in Soviet times post-Stalin, there appears to have been very little if any specifically Muslim disaffection. The CARs were essentially thrown out of the USSR, for instance, where the elites and most inhabitants would have been happy to remain within it (unlike, say, Georgia and the Baltics). Not great recruiting ground within the Soviet Union for Islamist terrorism (indeed, most former Soviet areas are still generally not that great recruiting grounds for radical Islamists, with the notable, though very conspicuous, exception of Chechniya).
 
More generally, it is important to stress that there is was no such a thing as international Islamic terrorism anywhere, against any target East or West,at any point during the lifetime of the Soviet Union.
There was international Palestianian (and other, Kurdish for example) terrorism, yes, but it was not, in any way, shape or form, "Islamic" (except in the sense you could say that the IRA in North Ireland is "Christian Terrorism", which is rather immaterial).
And there was plenty of "Islamic" armed political movements, mostly enganged in local insurgencies, both against foreign occupation (Afghanistan, Hamas in Palestine, South Lebanon, etc.) and trying to overthrow local regimes (such as the group who killed Sadat, and the guys who made all that fuss in Mecca in 1979). But none of them was operating at international level or against international targets, as far as I know. That began with al-Qa'ida, AFTER the fall of the Communist regime in Afghanistan it had been originally created to fight (there are also some attacks against Israeli interests that have been attributed to Hezbollah or related groups, which count as Islamic of course; but that's also in the early nineties. And of course, the Islamic resistance in Lebanon had no problem whatsoever with the Soviets).
So, the "weakness" of the open Western societies does not seem to help any explanation. Russia blipped on the militant Islamist radar more or less when the West did, that is, when there were international Islamist terror networks. Which effectively appeared in the early nineties, after the fall of the USSR.
 
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