Normally is because most (good) guitar players can easily
play most bass lines,ideed in guitar you need more
knowledge of harmony and how to use it,normally you'll
need more ability and skills to develop a good level when
you perform,besides in orderer to get good speed and
be very articulate and clean you need to work a lot in many
different techniques to master an electric as well as an
acoustic guitar.On the other hand I totally disagree with
those who consider that a bass is easier to play;if you really
want to play very good any instrument you have to practice
so,so,so,much;the only difference between those who play
these two instruments is that normally guitar players are
better recognized,but most guitar players don't have the
same notion of rythm and creating a real solid foundation
for any song and style as most bass players do.I am a full
time and lifetime musician by the way(guitar and bass player)
Albert.