Because the speed is what distinguished it. Chasing CoD or BF is a fool's errand.
The game was already (for those people) "CoD with mechs." So slowing it down to that basically gives them what they wanted.
The major problem with Titanfall 1 is (for PC) two fold:
-Lack of content (compared to BF4, CS:GO)
-No server browser
On consoles the Parkour was an issue for most "casuals"/mainstream.
If people wanted Tribes speed, they would've went with Tribes.

But, you're right: Chasing that audience is a fools errand, HOWEVER keeping the audience they have right now won't be too successful either given the game was dead and buried under six feet of dirt within a month of it's launch. Maybe more content would've helped with the Sanic speed retention. But I kinda doubt it.