Question about pink floyd- echoes the very best of?

i have a lot of pink floyds albums either on cd or vinyl. i got this collection to listen to stuff i either didn't own or hadn't upgraded to cd. my question is why are some of the bands better known songs missing? doesn't bother me much since i have the wall and wish you were here on cd.
obvious missing songs:
on the turning away (should be there for sure)
have a cigar
welcome to the machine
mother
young lust
run like hell
take it back

 

I believe all the band members, including Roger picked the list of songs on the Echoes release.
I think Dave was pushing for Fat Old Sun on the release, but got out voted by the other three.

 

yeah i know there was some democratic way of doing it but i don't think "on the turning away" was ever even put up for inclusion

 

Nor Take It Back.

 

i will at least say this is one of the best sequenced "best of" releases i've ever heard

 

This may provide some insights. Funny how two of the 'also-rans' that didn't make the cut later ended up on the even-more truncated best-of from 2011, A Foot in the Door.