Multiuser support in WLANs versus Cellular networks

About a year back I was reading about the physical layer design of 802.11ac networks, and I started wondering about what would be the similarities or differences between cutting edge cellular networks and WiFi networks. The reason I started thinking about this is because cellular networks seem to be gradually moving towards smaller cells (nano-cell, pico-cell designs), which are similar in RF footprint to regular WLANs.

So if these two types of networks are becoming similar, what could be the possible reasons they don't use the same physical layer mechanisms? One of the key things that stood out to me was the simultaneous multi-user mechanism used by both physical layers. LTE or WiMAX based cellular networks use OFDMA (orthogonal frequency division multipexing) while the second generation of 802.11ac networks use MU (multi-user) -MIMO.  OFDMA supports multiple independent users by separating them on subcarriers, while MU-MIMO uses a form of multiple beamforming to simultaneously transmit to clients.

From what has been usually discussed, it is believed that OFDMA is generally capable of achieving better isolation and MU-performance as compared to MUMIMO.
However, the possible reasons why this was not adapted in WLANs could be:

1. OFDMA does well on a licensed spectrum, where there are no un-expected interferers. The OFDMA phy relies on being able to setup MAPs (or schedules) so that the entire system co-exists without anything unexpected happening. MUMIMO on the other hand works well even in an un-licensed band where you could potentially have other interferers. This mechanism works well with CSMA which is the heart of 802.11 WLANs.
2. Another reason would possibly be backward compatibility. Switching to OFDMA would mean that older CSMA based WLANs wont be able to co-exist with newer WLANs. 

Both physical layer mechanisms are the means to achieving similar ends. However, both have their merits and operate well in their application domains. The key reason for the difference being that despite cellular networks becoming smaller in size, they still have a licensed spectrum on which they can operate.