Why MIMO antenna system performs better in multipath environment?
The whole point of MIMO is that you get multiple independent channels due to the multipath propagation.  In this case, one channel could be the direct line of sight, and one could bounce off of an adjacent building.  If the channels are sufficiently independent, different data can be sent down each channel.  This means you can have greater throughput than if you had just 1 channel.  And the more antennas you have at both ends, the more independent channels can be accessed, giving a higher potential throughput.