Because i hate the Appstore and never open it.
The MAS is growing on me. Having several Mac's in the household the convenience of not having to fudge with serials on a per app basis and being able to install all my purchased apps hassle free across Mac's is very welcome. So will be the additional security features especially as OS X is clearly being targeted more & more.
I now often re-purchase apps I already own and use a lot on the MAS if they become deeply discounted. Of course there are some cons ie some apps have to sacrifice a few features due to sandboxing and other constraints. Those I continue to use from the dev.