That's just normal. If you're running hundreds of clients, you can't go upgrading them willy-nilly, or their sites break and they complain. Also, in a bit of a chicken and egg scenario, why rush to put it on your newer servers if everyone is still used to using the previous version? Not to mention the inevitable security issues that will pop up for the first few months.
Expect PHP6 to take just as long to penetrate the market.
Expect PHP6 to take just as long to penetrate the market.