Because they couldn't decide which special effect to use.
The official excuse is that the phasers kept remodulating, the phasers kept being replaced, and so on.
Apparently the phaser that strangely fires from the torpedo bay is the best.
There's also the one that fires out of the Nacelles [on the Enterprise-D anyway]...
IIRC, the Enterprise-D has phaser emitters on the base of the nacelle pylons. You can see them in this picture: