The MU-MIMO scheme achieves higher capacity due to the fact that we are no more bouded to the number of antennas at the receiver (downlink case in single-user), which is the main motivation to go for MU-MIMO. In the case of MU-MIMO, the multiplexing gain when having perfect channel state information is bounded by the minimum of number of atnennas at the transmitter and number of users served simultaneously. Also it is known that the Dirty Paper Coding achieves MIMO broadcast channnel (downlink) capacity region. For more details, I recommend the following publication:
Weingarten, H., Steinberg, Y., and Shamai, S. (2006).
The capacity region of the Gaussian multiple-input multiple-output broadcast channel.
IEEE Trans. on Info. Theory, 52(9):3936 –3964.
In a MU-MIMO scheme, complex multi-user precoding is required to achieve capacity, this is why on system level only some simple schemes can be considered.