Software performance is the answer.
Apple can talk until they are blue in the face about how much better their operating system is than anything Microsoft has to offer, but the fact is that the guts of MacOS are a nightmare when compared to other modernized OS's out there. As far as I can tell, MacOS 8 has no protected memory and no pre-emptive multitasking. There's only so much that faster hardware can do for an technically unsound operating system. That is the biggest knock against the mac and so far Apple has not been able to provide any credible answer. There was the whole 'cooperative multitasking' fleecing.