Perhaps the most famous case of a prime-time network show being killed by pre-emptions hasn't been mentioned yet in this thread.
It was Season Two of the NBC version of "That Was The Week That Was", a weekly satirical "news" show which was an ancestor to the "Weekend Update" segment on "Saturday Night Live" and "The Daily Show".
Premiering in January of 1964, the NBC version of "TW3" (as fans called it; based on the original BBC version that became a big hit in 1962; David Frost hosted both versions) was quite successful for the balance of the season, and NBC renewed it for the 1964/65 season.
Moved to the Tuesday 9:30-10 P.M. (ET/PT) slot in September of 1964, "TW3" was pre-empted for several weeks in October of 1964 (according to listings in the Boston Globe), for political specials sponsored by Republican Presidential nominee Barry Goldwater (October 20th was the only week that month "TW3" aired; according to that day's Globe, Goldwater bought a half-hour on CBS in the same time period as "TW3"!).
The November 3rd edition was pre-empted for NBC News' live election-night coverage.
Supposedly, on the November 10th "TW3" (which I think was only the third or fourth time the show had been seen since mid-September), the show began with a clip of Goldwater's concession speech with an announcer telling viewers that "The scheduled Barry Goldwater Paid Political Program will not be seen tonight. Stay tuned for 'That Was The Week That Was'".
But all those pre-emptions (not to mention the move from Fridays to Tuesdays) cost the show it's large viewership. "TW3" lasted through the season, but was cancelled in the Spring of 1965.