Out of all the Looney Tunes cartoon compilations made for broadcast television, it certainly was the most diverse. Even local syndicated packages never did anything earlier than the mid-40's. And later cartoons? Forget about it. While I admit the mid-40's to late 50's was the cream of the Looney Tunes crop, to be able to watch those earlier and later cartoons you'd need to buy a Gold collection DVD.
Actually the pre-1948 cartoons weren't scarce, and only a handful of late-1960s cartoons were included on the Golden Collections. In fact, until last year official video releases of late-'60s shorts have been very scarce.
In the late-1950s, Warner Bros. sold its entire pre-1948 library to either United Artists or A.A.P. (and whichever other compaany bought the one that bought the library). At the time Warner no longer had the rights to the majority of black and white cartoons (for some reason the only exception was most of the Merrie Melodies produced by Hugh Harman and Rudolph Ising), re-obtaining the rights in the 1960s. Subsequently the various companies that owned the pre-1948 shorts were purchased by MGM, which was eventually purchased by Ted Turner. In 1997 Turner had a merger with Time Warner, making WB the sole owners of all the Looney Tunes and Merrie Melodies shorts once again (I often wonder what would happen if the two companies had a "divorce", but I don't think I have to worry).
MGM released many Looney Tunes VHS and MGM titles. In fact, "The Golden Age of Looney Tunes" laserdisc collection ended up including EVERY WB short that MGM had the rights to (except for "the censored eleven"). A number of pre-1948 cartoons have been included on many public domain releases as well (including the censored 11). In fact, none of the post-1948 shorts are in the public domain (despite this "Which is Witch" and "Gift Wrap" have been included on public domain tapes).
The post-1948 shorts had been split into many television packages over the years, allowing the shorts to be broadcast on many networks. For some reason (I assume to be able to show more shorts on their channels) the various television packages changed cartoons every few years. With Turner owning the shorts, the pre-1948 cartoons were heavily broadcast on TNT, TBS, and Cartoon Network (for some reason I didn't watch Looney Tunes on those channels often until after they started showing post-1948 cartoons, though I heavily watched LT on Nickelodeon and That's Warner Bros./Bugs 'n Daffy Show). After the merger, The WB began broadcasting pre-1948 shorts on The Bugs 'n Daffy Show and The Daffy Duck Show, and the Turner networks began broadcasting later shorts, while Warner Home Video began releasing pre-1948 cartoons on VHS. It was around this time that they set out to have Cartoon Network (along with Boomerang) be the only channel to show Looney Tunes. TNT, TBS, and The WB all soon stopped airing the shorts, and Time Warner let Nickelodeon and ABC's contracts run out. In fact, shortly after Nickelodeon stopped airing Looney Tunes, there was a commercial on the channel advertising the fact that Cartoon Network was now the only cable channel to air Looney Tunes, and CN aired a similar commercial after ABC lost the broadcast rights, promoting Cartoon Network as the only channel to air Looney Tunes.
I think that after these changes Cartoon Network was a little more diverse than Nickelodeon. The Bosko and Buddy cartoons never aired on CN (outside of a few Toonheads specials. It's a shame they wouldn't even show them on Late-Night Black and White), but the Harman-Ising Merrie Melodies were often shown on Late-Night Black and White. Black and White shorts were also shown on Toonheads and The Bob Clampett Show. CN also seemed to show post-1964 shorts as often as Nick did, with the main exception being the Speedy cartoons for a good period of time. And in the time since the Golden Collections the pre-48 cartoons have been released on other DVD collections, like the Superstars releases (though there was a brief period when WB wouldn't anything from before 1953, but that's changed now), the Looney Tunes Platinum Collections, and the recent Mouse Chronicles set which includes the entire filmographies of Sniffles and Hubie & Bertie, most of which are from before 1948.
Sorry, I hope I didn't turn this into a Looney Tunes thread, when it's supposed to be a Nickelodeon thread. But while Cartoon Network did show a little more than Nick, I think I actually prefer Nickelodeon's broadcasts of Looney Tunes better. I think the two channels weren't too strict on censoring violence (except for suicide gags).
I plan to post my thoughts on various Nickelodeon shows in a later post.