Because TP wasn't attached to any sort of defensive abilities or healing/sustaining abilities (except for abilities which actually replenished TP itself, go figure).

MP has its limit purely due to healing/raising/sustaining players. Without such a limit (and enrage timers), it becomes purely a war of attrition. When it comes to the DPS side of MP users, you'll notice that unless you stack spell speed or do abilities outside of damage (Raising with RDM/SMN, VerCure, etc) they do not ever run out. BLM is a fine example of this. Healers get double tapped for this due to A. Having to limit how much they can heal/raise/sustain at any one time to prevent a war of attrition and B. DPSing is stepping out of their main role. Sure, they can DPS but it isn't their main focus.

That's why, in a nutshell, TP was removed and MP was not, why MP has the limiters it has today and TP never really did aside from old school sprint.