The major reason? Sales. Microsoft never confirmed (or denied) that only 500 Kins were sold, but it's clear that the response has been completely underwhelming. Otherwise, why kill a project that was in development for years after just a few weeks? (And cost millions.)
A major reason it bombed, besides the weird, non-specific faux hipster marketing? Price. Verizon priced Kin's monthly service like a smartphone, even though it wasn't one. Even cutting the device price drastically didn't alleviate the high cost of the monthly plan. (The confusing Kin, a Windows Phone 7 Phone by Microsoft flustercluck branding didn't help.)
The few people that did buy a Kin will still get support from Microsoft, but the future of promised software updates is up in the air. It seems safe to say, though, Kin isn't going to evolve into the things we hoped it would. At best, we have to hope the things we did love—Kin Studio—will make it into Windows Phone in some guise.
Microsoft's official statement on the matter, for what it's worth, is vague but telling:
"We have made the decision to focus on our Windows Phone 7 launch and we will not ship KIN in Europe this fall as planned. Additionally, we are integrating our KIN team with the Windows Phone 7 team, incorporating valuable ideas and technologies from KIN into future Windows Phone releases. We will continue to work with Verizon in the U.S. to sell current KIN phones." [emphasis mine]
It's not the first major innovative E&D project Microsoft's slaughtered in recent months, but it's the first that's actually made it to market before being gutted. Verizon, for the record, says "The phone is still an important part of our portfolio."
All in all, still kind of astounding.