For those who don’t know Gravity is a popular Twitter client on the Symbian platform. When its developer Jan Ole Suhr was asked why he hadn't brought the app to Windows Phone, he responded that he had been kicked out of the Nokia Developer Program because he refused to write for Windows Phone.
Suhr feels that investing in writing his app for the Windows Phone platform wouldn’t be a financially correct move. He stated that he had made back his investment in the Symbian platform financially, and doesn’t feel he can do the same in the Windows Phone Marketplace.
From Nokia’s point of view it is important to have the important apps in the Symbian user’s app catalog available on the Windows Phone Marketplace in order to make the transition between the platforms easier for their user base.
Even so bullying developers into coding for Windows Phone isn't an option. There is still a large Symbian user-base, and if and until that dies away developers should have the option to code for that platform. Giving developers incentives like Lumia handsets is a great motivator, but given that the Windows Phone platform is currently awaiting revival though Windows Phone 8, you cant really expect any developer to transfer their apps to a dying Windows Phone 7 platform.
Developing for the Windows Phone platform is expensive compared to Symbian. Developers are expected to pay a $99 a year developer fee for access to the Windows Phone marketplace and it is wrong to push developers into coding for Windows Phone 7 given its current state.
source – MyNokiaBlog