What makes monks competitive with Paladins, if they are?

[Unoriginal]

That is a rather broad case assuming each of those is a sufficient condition.

A class that isn't desirable for a broad palette of cases isn't desirable by many.

If anything the 3.X Prestige Classes (and non-core classes to an extant) proved that. They only were considered attractive when a) you wanted the special gimmick or b) when someone found a way to broaden the use you got out of it.