Why can't CMOS simply be made of n-type enhancement mode MOSFETs and n-type depletion mode MOSFETs?
That's been done already. It's called "Depletion-load NMOS", as opposed to "CMOS" with the "C" meaning "complementary", i.e. both NMOSFETs and PMOSFETs.

The primary disadvantages of NMOS are(were):


Its power dissipation is higher than CMOS since the pullups never turn off (versus PMOSFETs).
Drive strength is asymmetrical due to high-side pullups versus low-side switches.