Edited Singles - 70s vs 80s

Being a vinyl collector, I've noticed that a lot of singles in the '70s and early '80s were short, frequently edited, versions of the songs. Pop songs continued to get longer as the '80s progressed, but towards the last half of that decade, it was more common to find 45s being the "LP version", and any edits being promo only. This is one reason why some edits you heard on the radio back then are hard to find.

My question: does anyone know why this was? While I liked having the long version on the 45, I'm wondering if record companies did this because single sales really dropped off during the later '80s and they wanted to try to encourgage sales by giving people the full version of the song. I think that's another reason why just about every top 40 hit at that time had a picture sleeve too.

Any opinions?

Grant wrote: This hasn't been my experience. In the late 70s, many, many edited 45s were different from the LP versions.

You're right about the late '70s, I actually meant the late '80s - sorry if it wasn't clear.

Using the timing printed on the 45, I've got the following (as just a few examples:)

Guns N Roses "Sweet Child o Mine" 5:55
Guns N Roses "Paradise City" 6:46
Don Henley's "Boys of Summer" 4:47
Don Henley's "The Last Worthless Evening" 6:05
Def Leppard's "Love Bites" 5:46
Bruce Springsteen's "Human Touch" 6:28
Bruce Hornsby "The Way it Is" 4:57

despite the fact that DJ edits existed for all of those songs. In the late '70s you were more likely to get that DJ edit on the commercial 45, where in the late '80s and early '90s (for whatever vinyl was released) it seemed that you were more likely to get the LP version.