Developers can stake more on one platform assuming it will generate more income than another. This could be for a variety of reasons; one console maker offers better incentives, has a track record of selling hardware, a franchise is attributed to a particular brand etc. As support for a platform grows, I imagine many developers jump on the bandwagon. They want a piece of the pie, and if the publishing giants of the industry support one platform, the smaller developers probably surmise that the risks are lower throwing their support in too.
They will switch platforms in a heartbeat if one platform begins to sink, or there is an opportunity to make more money elsewhere, or across multiple platforms. 'Loyalty' does not exist within this industry,or at least in any sane business it doesn't, but this surface 'favoritism' you speak of is most likely just due to a perceived stronger business environment on one platform.
