While more computer and console systems were designed with a Z80, the 6502 computers and consoles vastly outsold the Z80 systems.
The Z80 has more powerful instructions but they take longer to execute. The 6502 instructions are faster but you need more of them to do the same task.
In the end, a 1 MHz 6502 was almost equivalent to a 3 MHz Z80. The Z80 was available up to 4 MHz but often ran at 3.58 MHz) and the 6502 was mostly 1 MHz (eg. Apple II, Commodore 64) but in a few systems (eg. BBC Micro, Commodore 128) it was 2 MHz.
The Z80 has more powerful instructions but they take longer to execute. The 6502 instructions are faster but you need more of them to do the same task.
In the end, a 1 MHz 6502 was almost equivalent to a 3 MHz Z80. The Z80 was available up to 4 MHz but often ran at 3.58 MHz) and the 6502 was mostly 1 MHz (eg. Apple II, Commodore 64) but in a few systems (eg. BBC Micro, Commodore 128) it was 2 MHz.