I never understood why they won't raise the limit either.
I'm assuming it's because they don't want people finishing the next Prize Maps too early, and keep hoarding free bulbs?
They allow you to buy them, so they can make money. If you purchase them, you'll have well over the 50 bulb limit. They're just limiting the amount of free bulbs towards each prize map.
And there's no way to convert them to Rainbow Stars, and they most likely won't add this option, probably due to the fact that Bulbs are easily obtainable, and it would be like handing out free money.
I agree with raising the Bulb Limit though.
