People are comfortable with things that are familiar. When designing weapons, particularly for military use, muscle memory is strong contributing factor to design. For example, pilots initially had trouble with the flight stick in the F-16 because it responded to pressure, but didn't move. Eventually, it was redesigned to give a bit of the feel that pilots expected.
Noisy Cricket Type 1 phaser...
It's a very tiny weapon, designed to be easily concealed (as your image indicates). It is cradled in the hand rather than gripped like a modern firearm. There's not a lot of surface area underneath, and the traditional trigger finger is an important part of supporting the weapon. Moving the activation button to the top makes sense. You don't want to jostle your grip (and your aim) by moving a significant supporting digit. The grip makes sense here, because they're clearly trying to make a very small weapon, and a conventional pistol grip would result in a much larger weapon.
The larger phasers have a similar grip style and a functionally identical trigger mechanism, the top-mount thumb-trigger. This keeps training for the Type 1 weapon relevant for Type 2 weapons. The skills learned for one weapon are transferable to other similar weapons.
Of course, the Type 3 Phaser is significantly different, which serves as a good reason for why phaser rifles aren't used more often. Most people aren't sufficiently trained in them to carry them when you don't really need the extra firepower.