Why is it that some songs on a "greatest hits" CD are shorter than the original songs?

Question

For example, the U2 song "Where the streets have no name" is much shorter on their greatest hits CD than it is on the Joshua Tree CD.

Answer 1

Because the "hit version" can be a single edit played on the radio. If so, it is usually stated, especially since the 90's.

For example, Foreigner's "Waiting For A Girl Like You" was re-cut for the single version, shortening the beautiful ending. The album version is much smoother.

Other classic differences between album cut and single edit from times past:

Iron Butterfly - In A Gadda Da Vida (album: 18min, single about 4)
Dan Hill - Sometimes When We Touch (single edit cuts the very key emotional part of the last stanza)
Sweet - Love Is Like Oxygen (big length and bridge difference)
Andrew Gold - Lonely Boy (single cuts out a great guitar solo)
Gerry Rafferty - Baker Street (ditto)
Kim Carnes - Draw Of The Cards (album cut is so much better and haunting with the extra time)
Don McLean - American Pie (there's a 3:24 version, but the 8:23 version is the #1 hit)
Firefall - Strange Way (album has a cold ending with flailing flute!)
Pink Floyd - Another Brick In The Wall pt. 2 (#2 single actually has a guitar intro, album doesn't but ends longer)

There's more.

We won't EVEN get into annoying dance mixes.