There's p.enty off money in the enterprise market with great margins, just not in Apple's core competency.
Apple is a hardware company, not a software company. They're not going to sell something unless they can sell the whole package. Microsoft gets away with it because they sell software with insanely high profit margins, much higher than any tech company out there.
HP and Dell are the ones that get stuck in the middle, selling hardware with increasingly tighter and tighter profit margins to IT departments that continue to squeeze them harder every year. Apple doesn't want to deal with margin squeezing, especially since they are a hardware company.
Better for them to dominate other markets, and if they can be integrate with enterprise with things like Exchange support, great, that's enough. Aside from that then they can keep selling computers to high end media professionals and consumers.