Why no Soviet crypto-Jews?

Here's something I've been wondering about for a while: When the Spanish forced many Jews to convert to Christianity, and made being found practising Judaism a capital offence under the Spanish Inquisition, many Jews nevertheless continued the practice of Judaism in the privacy of their homes, to the extent that a few remnants of Jewish practice were still found amongst their descendants five hundred years later. However, after the Soviet Union outlawed the practice of Judaism, when seventy years later when the Jews were finally permitted to leave the USSR, they were all completely secular. Why were there no Soviet crypto-Jews (or at least not any I've heard of)? A century ago the Pale of Settlement was chock-a-block full of very observant Jews, and Vilna was called the Jerusalem of the North. Why was there no trace of this left after a mere seventy years?

This is something I haven't heard anyone else discussing, so I don't know what the answer is, but here's my guess: During the Czarist period, Jews were not allowed to reside outside of the Pale of Settlement. When the Soviets lifted this restriction, it was the younger, less observant Jews, who moved elsewhere. (This is something I learned from the book Through Soviet Jewish Eyes.) After all, at the beginning there were no synagogues in Moscow and other big cities, no shochtim or kosher shops, no eruvs, etc. The more observant Jews stayed where they had been—and were, with a few exceptions who were able to escape from the Soviet Union, wiped out by the Nazis as they swept like a wave across eastern Europe. The ones who survived, from whom the later Soviet Jewish population was descended, were the less observant Jews who had moved east from the Pale of Settlement, and ended up east of the furthest the Nazi occupation reached.

Or at least, that's my guess. What (if anything) do you think of it?

[ETA: Click on the below link to see dhole's response on LJ.]