If you really want plasma weapons in Star Wars, look no further than the Vong - plasma cannons were their primary armaments. However, Vong biotech never really pretended to make much sense nor were the inner workings of it explored, but that is not the case for normal Star Wars tech considering that you can find tech manuals that break down which parts of a laser cannon are what.
If you want a real-world explanation, I suspect that laser guns simply sounded easier to understand and cooler than plasma guns if the idea even was floated by the writers at all. If you want an in-world explanation, with a tiny dose of my layman's understanding of real-world science behind it...
Lasers are relatively simple technology - they are simply light waves formed into a coherent beam with minimal or no scatter, something we've been able to do in real life for decades (and new US warships are being deployed with laser weaponry -
no joke). An ideal laser would have almost no range limit to effectiveness in a true vacuum, if all the waves are travelling perfectly parallel to each other (and if space wasn't curved). Plasma, by contrast, is a very messy substance. A plasma weapon would essentially be firing a semi-liquid cloud at the target - an electrically charged, very very hot cloud to be sure, but a cloud, which is going to disperse relatively quickly after leaving the weapon due to the ionized state as the charged particles repel each other without external magnetic fields counteracting them, hence the short ranges reflected in most sci-fi fantasy plasma tech that I'm familiar with. Now, Star Wars lasers aren't true lasers, as far as I know - like Star Trek, most Star Wars tech actually fires charged particle beams (light bouncing around tibanna gas that excites the molecules, causing them to shoot off charged particles, mostly electrons - and no, I don't have a source for this handy
), rather than simply light, but charged particle beams act a lot like lasers in transit, maintaining cohesion over greater distances.
And there simply aren't that many advantages to be gained by using plasma over lasers or charged particle beams - either of the latter can cut through solid materials just fine as well, while having longer range due to superior beam cohesion. In raw materials, a basic laser requires a power source and some mirrors, a charged particle beam adds only a medium to run that light source through to excite, while a plasma weapon would require ionizing furnaces capable of withstanding the heat as well as multiple electromagnets to contain and accelerate the plasma itself - and a general rule in mechanics is that the more components and the more complex they are, the more that can go wrong. In a war machine that's going to be dealing with wear and tear, you want as simple and as durable as you can get while maintaining effectiveness. By my understanding, the upside to plasma weapons would be that the overall energy costs should actually be significantly lower, which seemingly applies well to the Vong-normal tech dynamic; the Vong wouldn't want their ships burning calories faster than needed but their ships could heal damage on their own and would require less maintenance, whereas the NR and Imps could just swap in new fuel cells as needed but even with droids damage repair and upkeep take up more time/effort.
If someone is better versed in the physics and wishes to correct my layman's interpretation of things, I sincerely welcome it, because I'd love to understand the differences better and, honestly, Wikipedia's a bit dense at times.