Because of HDMI "handshaking." Judging from the response by the Monoprice Tech Support person, the device does not act as a buffer between the source and the TV, and the TV tells the source that surround sound is a no go. With a fully capable surround receiver (with the source hooked up to the receiver's input and the HDMI output of the receiver hooked up to the TV), the receiver communicates with the source separately from the TV, and the receiver tells the source that surround is fine.
Now, I am basing this on that one reply by Monoprice Tech Support; I have no direct experience with that device at all. I suggest you contact Monoprice directly with your question, and refer to that answer on their web site, to make sure that the person responding to you has thought of that, and either will correct that if it is wrong, or tell you what I have told you already.
People have run into the same problem with little switch boxes for having 2 HDMI outputs to try to get 3D on their TV when their receiver cannot handle passing a 3D video signal (that is, they hook up their 3D BD player to the input of the switch box, and the two outputs of the switch box are connected to the receiver and the TV). But if the box does not act as a buffer between the things in question, the receiver tells the source that it cannot handle 3D, and so 3D is not output to the switch box. They can avoid buying a new receiver by buying a BD player with 2 HDMI outputs that communicate with each device separately, but if their BD player has only one output, then they have a problem.