Because to me Mos Eisley (the pre-SE version at least) felt much more like a real place. It was quiet and sparse, and there weren't tons of creatures and droids filling every inch of the screen and doing cute gags for the camera all the time.
The same goes for Yavin, and Hoth, and even Endor. They felt like real places you had just dropped in on (and not just because of the lack of CG either).
Again, all of those planets are Outer Rim. Of course they are going to be quiet and sparse...
We never physically go to Alderraan in ANH, but I would imagine that would be more in line with what we see on Coruscant (on the extreme, of course) and to a much lesser extent Naboo (which is in the Middle Rim) in TPM.
It's quite literally the difference between Manhattan NYC and a tiny little town in Nevada.