Funny enough, here in PHL, it used to be the opposite problem. Back when US was still in the *A and a UA codeshare partner, it was almost impossible, without using the multi-city option, to book a ticket on UA between PHL and certain cities (such as LAS and MCO). The search option would only show the US flights.

Also, for some reason, since the US merger with AA, I can now see the LH flight out of PHL come up when I search for PHL-FRA. That also was hard to get in years past.