I don't have the ability to give you a complete answer, but compared to the countries you list in the title, there are some things that set Yemen apart.
First of all, it is an Arab country. All the other countries you listed are not. Arab Socialism was a different sort of phenomenon than Soviet and Warsaw Pact Socialism, which of course is different from Chinese Socialism.
There is the direct proximity to Saudi Arabia to consider, also. Saudi Arabia, particularly after 1979, where the Kabba was seized by extremists, is a radicalizing factor.
Most of the countries you listed were also communist from 1945 and onwards, dominated to some degree by the soviet block, or directly part of the Soviet Union.
Yemen had a revolution in June 1969. Those are fundamentally different situations. The Yemeni Socialist party was not a Comintern institution, and wasn't born born before the second world war. Additionally, it was an at least partially nationalist movement.
You could compare it to Vietnam, for example, which had a very different form of Communism to China or the USSR. Vietnam has had different repercussions of socialism than Russia, China, or, say, Romania.
Yemen was also only half socialist. South Yemen was Socialist, but North Yemen was not. There was a civil war, and the socialists lost.
Frankly... I think this might answer your question, but I can't say for sure. Especially since I'm not even convinced your basic assumption is correct, i.e. that there was no drop. It could have been a smaller drop, or been mostly in the South.
And could it be argued that the Houties are slightly less motivated by religion that Al Qaeda... say? This might be apples to oranges.
Sources: http://www.globalsecurity.org/military/world/war/yemen1.htm
I have removed all comments not related to fundamentalism in the People's Democratic Republic of Yemen (South Yemen).
It's not totally accurate to say that these countries have had no major terrorist activity. Uzbekistan is one I one for certain has, as the Islamic Movement of Uzbekistan was (and still is) considered a serious isue hy the authorities. Now how much of it is real and how much is manufactured fear by the Karimov administration to help consolidate power is u for debate and really to recent to be discussing here, but it certainly was something the Uzbeks dealt with post-independence.
I would suggest that if you consider Chechnya to be a state of some sort, then it would also be included with Yemen as having produced terrorists.
Modern militant Islamic fundamentalism, the Salafi and the Twelver kind, received significant resources after Saudi Arabia and Iran, officially exported it starting in the early '80s. They did so by building mosques and madrassahs in impoverished countries, by providing students scholarships in specific schools of Islamic jurisprudence, by financing local extremist political parties and/or public projects etc. These tactics worked in states with either weak central governments or pre-existing separatist or radicalism sentiment. So, the astonishment isn't that only Yemen (actually Tajikistan also) developed such movements but why didn't all the failed states? Kosovo, Kazakhstan, Uzbekistan, Turkmenistan and Azerbaijan (after Nagorno-Karabakh) all either have strong central states or lack a major separatist constituency (though Kosovo & Bosnia are seeing adverse effects after the participation of Salafi irregulars in their respective conflicts). Tajikistan has the former but not the latter and saw an Islamist insurgency. Yemen has both the former and the latter. Other communist areas such as Xinjiang in China, Caucasus countries absorbed into Russia (Chechnya/Dagestan etc.) all are suffering from the results of this strategy.
But, why limit ourselves to fundamentalism/terrorism/insurgency of a specific kind in Communist countries? Bosnia saw Serb and Croatian terrorism/fundamentalism and various Orthodox/Catholic churches have active involvement in conflicts of the former Iron Curtain countries. In these countries, you don’t hear the conflicts labeled in religious terms unlike in Muslim countries because ethnicity or nationalism is not a primary identity in most Muslim countries while perhaps it does seem to be in Slavic countries.
Generally, it would have been ridiculous to connect these disparate conflicts into one theme if not for the fact that there truly is an underlying catalyst/enabler, which is the active financing by theocratic states.
Maybe I should have said "adherence" or "strict observation" rather than fundamentalism. But please don't be stuck in the terminology, I believe you all get the point.
Can't talk for all those countries but in Uzbekistan Stalin carried out a massive operation against imams and other muslim leaders that either opposed or could oppose the USSR and communism. Many were deported, some were disposed of, others sent to reeducation camps and after coming back they became informants for the NKVD/KGB, reporting on any suspicious activity from the people in their communities.
The Thaw relaxed things a bit but only a little, being an outspoken muslim against the atheist USSR was still a death sentence. On the other hand Yemen didn't have a Soviet-style socialism but Arab socialism which had a strong religious component and thus didn't carry a campaign of oppression against religious figures. At the same time it only lasted a short time in which socialists didn't have control over the entire country, there was a civil war going on and they eventually lost.
Also, as a point to think. While Chechnya is not an independent country, it an ex-communist substate where a really large part of modern Islamist extremists come from and originate. This is a caveat to your point.