I've always wondered why the only TAS I have found is the prologue TAS. Is that the only one because of how time-consuming and tedious it is to do a TAS? Is it another reason?

It's a couple things. The first barrier is the software used to TAS Mirror's Edge; it isn't publicly available by the mod-maker (to my knowledge). Second, there's some controversy surrounding that particular Prologue TAS, as Faith seems to move in impossible ways. To be more specific, some runners speculate that the method of TASing used to produce that video resulted in a recording that couldn't actually be done by a normal instance of the game engine. To my knowledge, the speculation is just that—speculation—since nobody has the software to try to replicate the TAS. Lastly, TASing is time consuming, which is another factor.

I would love to see a proper TAS of Mirror's Edge someday, but I don't think we'll get there anytime soon.