Microsoft cannot promise developer incentives that actually scale. They have run several "hackathons" and "app contests" and things with eye-popping prize money, like $100k and whatnot.

But for most people, supporting an additional platform for any duration more than a year can quickly cost upwards of that amount. With the tiny marketshare today, and nothing but hope that it will ever grow - hope that has not come to fruition in 3-5 years - it will be a net loss very quickly and also have diverted resources from the other aspects of the developer's companies.

If they could perhaps offer scalable incentives, like "support and consulting services worth $2000 per month" or "we pay you $0.05 per download (even if your app is free)" or "we'll market your app alongside WP marketing" then perhaps. Perhaps it might be functional developer incentive.

Disclaimer/note: As a cofounder of a company built around an app that Microsoft tried to woo us to port. We toyed with their APIs for a bit, but it was going to be a support nightmare with our hardware and sensor requests and we decided against following through.