Because any budding attempts to repeat the process would be eaten by the evolved results of the *first* success.
This "only once" rule sounds suspiciously like a darwinist version of a "god of the gaps."
Only if you don't bother to think it through.
Inded, it goes against all rationality to suggest that it did not happen often.
"Often"? Only if it were "easy" to occur. The best understanding at this point, however, is that we're probably pretty lucky it happened at all.
How have you determined with such precision how "easy" (or difficult ) it was to occur?