The first time I've got my hands on a Z80 clone it was like "my precious". For example AY-3-8500 was not unheard of but was unobtainium in every respect.

If you're considering clones in East Europe, I think the real reason was economic. It was useless to have a complex circuit when the devices were not available. For this reason, most early clones of Sinclair Spectrum used only 7400 series circuits around Z80.