There are a few reasons why UCLA has not played certain teams in its long storied football history.
Geography: Some teams are simply too far away to make a regular-season game feasible, especially when traveling far distances for football games were not as financially responsible for programs like it is today. For example, UCLA has never played against some Big Ten and ACC teams, as they are all located in the Midwest or East Coast.
Conference affiliation: UCLA has been a member of the Pac-12 Conference since 1959, and the conference has a scheduling agreement where each team plays nine conference games as opposed to eight like some other conferences.