I think part of it is that it's just very direct and visceral in a way those other things usually aren't. Based on narration, our character is aware that they're losing something as they use the tadpole. Making a deal to sell your soul in the future, worshipping an evil god, those things are more distant, more of a "someday" problem. Or with the gods, a case of religious zeal and fanatacism that doesn't necessarily have to end badly for you. And even if it does, you've been indoctrinated to believe that such an end is a worthy outcome for your devotion. As for necromancers, they're at least theoretically in control during the whole process if they became a Lich. With the tadpole, you have no control over it, no understanding of how it works, and you do know that using it immediately costs you something whether you like it or not. Using it in moments of desperation are the clearest justification to my mind.

I genuinely think if they didn't make it so immediately obvious that you're losing yourself every time you use the tadpole, if it weren't so visceral and undeniable to the player in particular, using the powers would be a broadly more appealing prospect.