Why do we demote /fail series to 75 LP but promote to 0 LP?

Would it not make more sense to demote to 100LP? I can have 99 LP but any win will always set me at 0LP on promotion, so why does failing a series or demoting drop you at 75, more or less making you need to win 2 games at least to be in a series again. If 100LP is too much, then set it at 99 so we only need to win 1 game to re-qualify.