I don’t know for certain, but my two biggest guesses would be cost or weight. Convertibles typically cost more than hardtops, and not because theyre sportier. The mechanism and parts for the convertible top cost much more than not having them. Theyre also much heavier than a hardtop.
The other consideration is space. Hybrids usually hide the battery behind the rear seat in the trunk. This is exactly the same place convertibles collapse the top into. If you have a battery there, you basically lose the entire trunk if you make it convertible.