It's more simple than that. The developer base doesn't exist for Windows Phone yet because Microsoft didn't really support it's own system. When they brought out WP7, they relied almost solely on the hardware producers to sell it and put it out there. Why would somebody want to spend time, effort, and money on a platform that the owner didn't even seem to put much effort behind?
Now that they apparently are making the effort to put Windows9 and tie in Windows Phone 8 along with Tablets and other portables, perhaps now, developers will begin to see some payback for their efforts. They did so well, for so many years with Windows Mobile.
They dropped the ball when they abandoned that OS for the new. I think they may have learned something from that judging from how they are handling the WP8, WP7.8 changeover. They could have very easily done the same thing to everyone who'd spent time/money on WP7 but they at least are doing something to keep it going for a little longer until WP8 has time to develop and grow.