Reason one: Safety
LH2/LOX is a high-energy fuel
so its explode also like that...
Reason two: Boil off
LH2/Lox are super cooled to be liquid.
ones in a Tank the LH2 (hydrogen) start boiled.
and the hydrogen gas has to be vent out
or the Tank explode because of overpressure
so after some days in Space the LH2 tank is empty
of course we can install better Isolation and a Refrigeration unit for Hydrogene
but that increase the mass of spacecraft and so Rocket to launch it.
NASA look for LH2/LOX Apollo proposal
but there were only the edge of possibly technology in that time (1961)
and went save with N2O4/UDMH
IMHO that Save Apollo 13 Crew.
had they LH2/LOX Propellants wen the LOX tank explode
they would be death now...