United generally tries to limit the length of available connections, both as a response to typical customer demand and so they can charge people more if they want to include a stop in their travel to visit an intermediate city, which United would rather sell you as two separate tickets for additional revenue (in part they can do this because if you're connecting anyway, other airlines are competing to get you from A to B via various C-connecting cities; but if you want a specific connecting city they realize they can charge you more). Oddly they are more flexible with award tickets, where you more typically will see very lengthy connections.