there definitely are reasons for "why not", such as requiring ongoing development time that could go into Proton's many already existing products instead.
but also, what's wrong with Signal? or rather, what could Proton do better than Signal that would actually provide meaningful value and not just further fragmentation in the messaging app market? considering most people still just use WhatsApp or maybe Telegram because everyone else does, too, and cross-platform messaging does not exist to my knowledge