One of the reasons is the advertising budget Microsoft has for this. Half a billion dollars is nothing to sneeze at. You'd be hard pressed to find an objective site that has written a negative review of the new phone, so a solid platform with ton's of money guarantees there will be some not insignificant sales in the coming year. You can read the rest of the article at their site for other incentives for writing for the platform.
One not mentioned is the cost of doing so could be lower than it is for some of the other platforms out there. Microsoft required all devices meet their Chassis 1 specification, which means the developer just needs to target that and the app will run on all devices. This is a stark contrast to where Microsoft found themselves in recent years with Windows Mobile where you had touch and non-touch screens, a myriad of different screen resolutions and hardware and UI customizations by OEMs and carriers that would make any developer run screaming for the hills.
This scenario still exists, but not for Microsoft's new platform. It is Google that now has this problem with Android. Adobe Air was recently released for Android and one of Air's most popular desktop products, Tweetdeck, is being tested on Android now. It is shocking how many variants of Android devices there are. According to a PC Pro article on Tweetdeck's progress, Tweetdeck's developers call Android "extreme fragmentation." With a beta group of just over 36,000 users, there are 244 varieties of handsets and over 100 different versions of Android!
Now, I know Google hasn't released 100 different versions, so this must be a combination of the various versions from Google coupled with manufacturer customizations, many including full user interface replacements. There also appears to be some smaller builds by companies or people customizing their own version. Tweetdeck is up and running, but much of that has to do with AIR being the single common factor across all devices.
The fragmentation of Android isn't insurmountable, but it is an obstacle. One developer relayed an anecdote where a multimillion dollar project was held up due to implementing quality assurance on such a varied landscape.
Developing for the Blackberry is a similar situation, though on a much smaller scale. There are various resolutions, touch and non-touch screens. Even versions that support touch have to distinguish between a Torch and Storm.
Microsoft saw this as a weakness in its Windows Mobile and took steps to virtually eliminate it in its new platform. As a developer, that has to be a checkmark in the "pro" column for developing for Windows Phone.