I think both reasons are pretty obvious. For a stereo system you're always going to want matched speakers. You would never buy two different right/lefts in a home theater system. You'd also want a center channel from the same line as your right/lefts. Sure apple could work around it or let you do it anyway but why would they bother spending money to build something that isn't going to be great anyway.
On the wifi front, it's using an older wifi b/c it's using Apple Watch tech that probably only supports that Wifi. Plus it's not like streaming audio requires that much bandwidth that you need newer tech. It's fine and does the job.
To a CUSTOMER, "WANT" is what the consumer "WANTS", not some self-serving decision made, with lipstick added to hide the hog, as in the market twaddle used to justify the Apple BOTTOM-LINE-optimized design.
Yeah, it's not OPTIMAL, but what's even less 'optimal' is not being able to upgrade to the new model one-at-a-time, which allows auditioning by the CUSTOMER to see if the change is necessary.
That 'necessary' question is the one Apple has apparently decided in it's OWN interest, not the CUSTOMER'S.