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.
Its not as simple as moving the battery to the floor. Hybrids use the same chassis as regular ICE cars, which are not designed like EV chassis. Designing it like this would defeat the whole purpose of making a hybrid. At that point you should just make a full electric.
They exist, but they're expensive. The cheapest I see is the Mercedes AMG E53 Cabriolet for around 80k.
The reasons why they're expensive are touched on in another post: hybrids are heavy because batteries are heavy, convertibles are heavy because you can't use the roof for the car's structure anymore, so a convertible hybrid is extra heavy. Solving that engineering problem makes them expensive.