Because if that were the case then ship based battles would be over very fast indeed.. One can only guess the a TypeX phaser mounted on a ship is waaaay more powerful then a hand type. Yet, we've many many time where the phasers do no damage whatsoever. Now you could argue "shields" took care of it. But once those are down you'd think ships would look like swiss cheese. Or, just be "gone" all together.
And indeed they are. Shields block phasers; if there are no shields, the hull will go away, as in ST2 where Khan cuts into the Enterprise and Kirk into the Reliant. The behavior of the narrow phaser beam here is similar to that of high-setting hand phaser beams applied against walls: it creates a tunnel of emptiness in the target, rather than spreading the go-away effect from the impact point to the entire target. It's probably a mixture of parameters the user can select and physical laws that govern the propagation of the disappearing effect in various materials. Or whatnot. Now, good phaser hits in the older shows quite often made ships "gone" altogether, although this generally involved a big fireball and implied that the narrow hole made by the phaser had reached something vulnerable. Modern ships just get Swiss-cheesed, such as in the DS9 battles. Then there's the ability of the Defiant's "ablative armor" to resist phaser blasts to some degree. But the very name suggests that the armor indeed goes away when hit! It would be rather easy to imagine a material that welcomes the disappearance effect of a phaser beam and allows it to propagate - all of a tenth of an inch to the next layer or cell in the armor, where it meets a phase border it is reluctant to hop (much like the hand phaser vaporization effect prefers not to hop from an intruding Klingon to a starship floor). And the technobabble from "Past Tense" suggests that the ablative armor isn't exactly physical matter, but something more complicated than that. Yuta might have been made of stern stuff, just like the rock that takes Level 16 to drill through in a practicable time. But Yuta is a small target, and anything that has any drilling effect at all should be done with her quickly enough: the extra oomph for speed that Worf selects in "Chain of Command" would not be needed. Yuta certainly is no starship wall! Timo Saloniemi
Indeed, however there is no reason to assume that all matter is effected the same by a phaser at a level. Thats why they goto 16. Just because you and I couldn't take a blast at 8 doesn't mean she couldn't take a hit or two.
I figure that with the approach of the Dominion War, and the resulting more militant stance of Starfleet they were replaced with the type II as the standard sidearm for more power. The type I would be more for concealed carry, diplomatic security, etc.
They practically disappeared long before Starfleet ever found out about the Dominion. I tend to think it's a battery thing - the only reason to carry a Type 1 over a T2 is if you're trying to hide the fact you're carrying a phaser. But, y'know, any alien race worth its salt will scan for phasers with a tricorder-type instrument if they're concerned about it, so there's little point unless you're dealing exclusively with a pre-warp culture. Which you shouldn't show a phaser to because of the Prime Directive. So, ultimately, where's the advantage in carrying a mini-phaser that has a smaller battery and thus fewer shots over a not-exactly-huge standard phaser?
What's the advantage of a cigarette pack -sized mobile phone over a not-exactly-huge pocket calculator -sized one? Beats me, and the disadvantages are legion, but people still buy those. Type 1 was a "civilian" weapon in "Devil in the Dark", not a concealed carry one. We might assume it has been sized and shaped to be comfortably carried 24/7 without needing a special holster or belt, because civilians would complain about the angularity and extra weight of Type 2. Also, our heroes might well more or less permanently carry a Type 1, unless special circumstances dictated otherwise. When were they ever caught unarmed? It's more a case of them sometimes leaving Type 1 behind (or inserting it in its usual place) when packing a Type 2, and then being deprived of that due to a plot twist. Timo Saloniemi