The drug stores allow shelf space for toothbrushes in direct proportion to
the amount of advertising done by the manufacturer on each one.  The drug
store is only trying to capitalize on the marketing done by the toothbrush
manufacturers.

I often will still see about 15-20 Butler or Oral-B brushes displayed with
straight handles, straight bristles (all of the same length and direction).
They are getting harder to find.  I personally prefer the Butler #411 as it
has a straight handle--so that the patient actually can picture where the
head of it it.  It has straight even bristles--so you can brush with a
"whisking" motion instead of a back and forth one.  However, keeping the
brush in your mouth longer makes a lot more difference to how much plaque
you remove than the toothbrush design does.