Oneconnect and http profile - why recommended, and not mandated?

The same SOL alludes to one reason. OneConnect can be used for protocols other than HTTP. However there are some guidelines on protocol behavior that would not be suitable for OneConnect, which are listed in the SOL. Examples are listed in the 'Recommendations' section of the SOL.

So the 'http' profile should only be used if the traffic handled by the virtual is http traffic. OneConnect could be used for other (non-http) protocols to load balance separate requests inside the same TCP connection, instead of load balancing the TCP connection itself (i.e. all requests in the connection will go to the same pool member).