This includes The CW's predecessors, UPN and The WB. I know that the immediate reason (https://www.quora.com/Why-doesnt-Fox-have-the-same-nightly-broadcasting-times-as-the-other-big-3-ABC-CBS-and-NBC-plus-network-news-at-6-30-with-an-anchor-They-have-3-hours-while-Fox-only-has-2-hours-of-prime-time) is that their charter affiliates in particular, have great success at airing their local nightly news telecasts in the 10 p.m. timeslot, since they aren't directly competing with the "Big Three" network's newscasts.

But bare in mind, that Fox has flirted with first-run programming at 10 p.m. in its early years. For example, Fox back in the early '90s aired Flying Blind (http://tvtango.com/series/flying_blind/episodes), a kind of proto-Dharma & Greg sitcom starring Tea Leoni in the 10 p.m. slot.

I read that Fox's initial reasoning for only programming for two hours a night (or three on Sunday evenings), is to avoid numerous restrictions the FCC placed on "networks". So they would only broadcast under the minimum number of hours to be considered a "network" by the FCC. Basically, anything over 15 hours of prime time programming meant that you were "legally" considered a network by the FCC.

Before Fox and the CW existed, a number of large market independent stations had newscasts that are still existent today. The first was Channel 5 in New York that always opened with the disclaimer "It's 10 O'clock. Do you know where your children are?" In the Twin Cities, KMSP's Fox 9 News gets high ratings.

Fox is just kind of a mini-network - no nightly newscast or late night show. Although they did have a late show at one time that aired at 11:00 pm hosted by Ross Shafer and others. But apparently it was too difficult competing with the mighty Carson and they pulled the plug.