The reason is that it's hard to guarantee that two different sound cards will play in step with each other.
This is soooo true. I ran into this problem when recording through two different sound cards. I was recording a concert with Cakewalk Sonar. My mics were recording the room through a USB soundcard while I was running a feed from the soundboard into my laptop's line in. I didn't anticipate that the tracks would not be synchronized and one would drift slowly off from the other! There must be some weird thing going on because I was even using identical sampling rates on both of them. Nevertheless, the drift was slow but inevitable.
In the end, I had to go through the entire file and manually stretch it every few minutes before it got too far out of synch. Strangely, the rate of "out-of-synch-ness" seemed to vary. In other words, if I stretched the entire audio track so that both the beginning and end of one card's were in synch with the other card's track, parts of the middle were out of synch. Huh!
What you might do is get a matrix switch for your speakers or your line-level signals so that you could put any one or more MC zones into any one or more speaker zones. In other words:
MC plays Zone 1 -> matrix switch -> speakers zone 1 and 2
or you configure the switch this way:
MC plays Zone 1 -> matrix switch -> speakers zone 1
MC plays Zone 2 -> matrix switch -> speakers zone 2
Don't even know if anybody makes such a switch, and if so, is it remote-controllable? And how much is it??? But that would definitely address your problem.
Come to think of it, I know that they make distribution amplifiers that would do what you want. But they run in the thousands of dollars...