Because that's what Nissan wanted? The GA was never meant to be a high performance motor.
That is the honest to God truth. It really wasn't.
A full bolt on 1991-1994 Sentra with the GA16DE (cold air intake, header, exhaust, JWT cams) will still not even beat a stock SR20DE. That tells you right there whats up.
I have no hate for the GA16DE, I've owned a 93 Sentra XE and a 94 Sentra LE and it's a great economy motor, in fact great cars, but not meant to be up there in power.