Nothing really changes by taking away TP from melee, while taking away MP requires broad changes to enough classes that it would be far messier to implement. What do you do to limit the healer that just spams their most powerful cure over and over again? What about the Paladin who is now free to spam clemency or holy spirit? The blackmage who can now entirely ignore two of their three elements? Sure it wouldn't make a big impact for summoner or redmage, but there's enough classes where it does that I imagine the workload to overhaul those classes further wasn't worth the effort.

The only time TP came up in the entire game was if your AOE damage wasn't good enough to kill the adds you had pulled. It served no real gameplay purpose outside of that so it didn't make sense to keep it as a barrier.