It may be simple but that simplicity is its downfall. How many databases, compilers, modern games. OOP classes or even file handling routines do you see in assembler? Few or none at all. Writing assembly language is slow and painstaking work. You can easily get 10x the productivity by switching to C or even better in higher languages. Games were written in assembler back then because Basic was too slow. And C compilers were only starting to appear on personal computers from the late 1980s.

Writing an assembler for 8-bit CPUS like 6502/Z80 was doable; I wrote a 6502 cross assembler (in Z80) so a Z80 development machine could output 6502 in 1985. It got harder with 8086 and 68000 chips.