[quote name='silddx' post='1310326' date='Jul 20 2011, 02:20 PM']Why, Mac?[/quote]
It's the mechanics of the thing and how you play. Starting off on bass it's gonna be one note per string and maybe a lot of sliding about on the E string. With guitar you're gonna be learning chords (playing more than one note at a time) and maybe trying to learn a bit of lead. Of the two without great technique the bass is initially easier. Also without great ability, the bassist has a better of being able to play a part in a band.
But to improve as a bassist, because the neck is longer, the strings are thicker, and the distance between strings is greater, the bass can be more difficult to master - certainly on a technical level. As bassists develop as musicians we gain an understanding of the bass role with other instruments, so a good bassist will have an understanding of how the bass can affect and even control harmony and feel (I remember an article in BGM mentioning something similar saying that's why bass players can become great producers). A lot of good guitarists don't have this awareness, which is why it's often possible to spot the bass lines played by guitarists.
This all of course IMO and exceptions will obviously apply