Several reasons. First of all it has no range, you have to be on top of them to land it. Second, it has two hits, so if a person knows what they're looking for they can armour after the low as it will beat out command grab and Kano ball, which is your only real mix up off of it. Thirdly, if the opponent blocks high, the second hit will connect and the command grab will whiff (if you cock up the OS timing).
With D4 you get to tick on crouching opponents from much further out, and with D1 it's faster, so less time for you're opponent to react, and is+25 on hit, so you can follow it with: another tick, a pause and a raw command grab, b1, b14, b14-choke, f4, b3, b32, b31-choke, 1, 11, 112, 112 choke, cross over jip into one of the previous options, cross over jik3 tick into choke.