Walt Mossberg has an interesting piece explaining what he calls the Mac's second act. He makes a lot of good points about why Mac market share continues to rise. I agree with most of his points but I also think he left a few out. The rise of Windows happened at a time when most computer purchases were made by companies, not individuals. Back then the decision was driven by people that administrated computer networks, not people that worked at the computers being purchased. That's not true any more. (Or at least not as true.) Nowadays, a lot of people buy their own computers and are more discriminating.
Another reason for the Mac's success is that the Apple tax, is now largely fictional. Macs aren't as inexpensive as junk computers but their pricing is right in there for comparable Windows PCs. That wasn't true the first go-around.