Probably because they would also then need to clone the system software as well, a much bigger task for a much smaller (potential) market. Apple would have been much less likely to deal with clone makers than Microsoft was. While the system software was made freely available in the early years, it is unlikely it would have stayed that way if clone makers had been successful. For a brief period, NewTek did have a Macintosh clone with a 68030 and system software that was a hybrid of System 6 and System 7, with a user interface based on Motif. It suffered the same problems as the 1st - gen IBM clones as far as compatibility.