Subject: Why can rainbows only be seen at specific angles?

I used some known equations to calculate the angles to see rainbows. The first
rainbow's angle is found using D(a)=pi+2a-4B, where D(a) is the full angle
rotation of the refraction and reflection the sun's white light. The variable
'a' is the incident angle for light entering the rain drop, and 'B' is the
refracted angle. The angle to see the rainbow can be found by differentiating
this function to find the minimum deviation of the angle 'a.'

My question is "Why must we solve for the minimum angle of deviation?" Why can't
we see the light at any angle? Why does it matter what the total deviation of
the light is? If so, why don't we see, not multiple rainbows, but a wall of
color bands? What is the limitation behind seeing rainbows? 

Re: Why can rainbows only be seen at specific angles?