The other factor is kinda stupid, it's the aesthetics
For some reasons, I don't like the looks of missiles, I prefer beam weapons or mass drivers, though I'm probably the only one to do it like that...
Beam weapons, imo, are the coolest looking. Plus, depending on how many actual weapons are on a ship, it shows that many beams being fired. For mass drivers and for missle weapons, it may do the same thing, but it is not as noticable.
Beam weapon battles of various Terran Fleets:

2 Marshall Class destroyers and a Normandy Class defender bump heads with a Torian Fleet.

A Planetary invasion begins spearheaded by the Patton Class Battle Cruiser.

A lone Patton Class Battle Cruiser getting pounded by a very large fleet of enemy ships.

The Patton Class unleashing a deadly fury of beam power from it's forward undercarriage.