I think this subject can be answered differently by Songwriters and instrumentalists
Songwriters are concerned with the content of the written message. Generally taking something big and presenting it in the most condensed form while keeping the subject and the emotions covered. A long song with tons of lyrics can be a disadvantage because it might become too repetitive.
When I apply this approach to myself - a long song makes me feel that I'm not ready to cover such subject and I abandon those songs.
Instrumentalists can approach this from an entirely different angle - delivering emotions is stretched over time. Songwriter says "Sad" instrumentalist will have to go through 8 bars with modal interchange in order to convey the same feeling. There are cases which can be opposite but they are not that practical.
Another point from the instrumental perspective - the player's proficiency is showed over technical skill (solos, complex harmonies) and general delivery skills (putting the soul into it) so in a band/group with songwriters and proficient musicians, the songs will tend to be longer.
My best example would be campfire singing - the songs are the same but they are almost twice shorter since one person can deliver mostly on one thing in this setting. A counterexample can be a Blues concert with 3-minute songs - it wouldn't feel right since blues heavily demands the music to convey the spoken message.
Whoa, seems that I went on a long rant here, I'm going back to my Linux servers now.