The thing with 3rax reaper is that in TvT as long as you keep advancing your tech and buildings while also efficiently and effectively keeping reapers at bay (Losing less units and killing more) you can defend.
And how are you supposed to do that vs overwhelming numbers with just reaper FE, asssuming your opponent isn't throwing away reapers? How would you transition if you scouted after depot upon scouting 3rr? In my experience, gas first is one of the few all-round safe openers in tvt. I've tried to make any form of reaper FE work but you just die to 3rr unless the map is terrible for it and/or your opponent utterly incompetent with microing reapers.