I always felt it was more that the phaser II was supposed to be so powerful, one didn't really need a "rifle" version.
The reason I linked the images was to suggest that the golden, cylindrical core component of the pilot phasers can be easily spotted in the phaser rifle, only much longer and times three.
It almost seems phaser I is the analogy to the pilot phaser, while phaser II could be the analogy to the phaser rifle.
The production, however, probably didn't reuse the rifle because they didn't own the design and would have to pay royalties for a reuse (if they were allowed to even reuse it at all).
Apparently Gene Roddenberry stated something along the lines he didn't want rifles in the regular series, IIRC because of allusions to westerns (hmm..."Wagon Train to the Stars"?
Now either he changed his mind after "Where No Man Has Gone Before" or it was the excuse not to pay royalties.