Because MSE is a good, as is antivirus. but when MS said they where putting out a free AV all the antivirus companies about crapped themselves, so to keep them all happy, MS keeps their AV to a bare minimum so that the other AVs dont have a cow.
So the scans are slower, and you dont get hardly ANY options, but it works, most of the time. There is no such thing as a perfect AV, and I mean none. While one AV may do good now, they might not later, it is a constant cat and mouse game with the virus makers and the users in the middle are the ones who pay for it.
Shane