I booked a ticket that includes a segment on a United Express / Go Jet with explus.

However, neither the United web site when choosing seats during booking, nor calling the automated phone system afterward (which did not have an obvious way of talking to a real person) allowed choosing a seat on that segment (i.e. not allowed, as opposed to offering you a map showing no seats available). The United web site did allow choosing a seat on the other segment on a United 757.

Is there something special about those United Express / Go Jet with explus segments that is why a seat cannot be assigned?