Because he thinks and plays like a double bassist with the neck as vertical as he can get it - he once explained that most electric bassist construct their bass lines from the E and A string as their base, while a double bassist starts from D and G and moves to the deeper strings - and that he opted for the latter though he regrettably couldn't really play double bass as his hands were so small.
Wyman also liked to play in the middle portion of the fretboard (a lot of Stones songs are in B) - he would rarely play beyond the 12th fret or real deep/low, towards the headstock, he even avoided empty strings (probably because with his just-behind- or even on-top-of-the-neck right hand positioning, palm muting wasn't really an option for him). If you like to spend your time mostly between the 5th and the 12th fret, his bass positioning makes sense too.
Now that I play in a Stones tribute, I spend more time analyzing the man. I don't really slavishly copy his bass runs (neither does Darryl Jones btw), but I try to understand his approach. For instance, he had a thing for fifths. Where a lot of bassists would have played the root, he would play the 5th, even his bass run to the Satisfaction riff is constructed that way, he plays an A to the D chord accompanying the H/C#/D riff (after the initial E chord). Not very rock at all in sound, but in line with the Samba feel of the original recording.
Bill is severely underrated though Darryl Jones' style is more stadium sound-compatible & -effective.
