Eventually you hit a limit where you simply can’t create enough pressure to propel the projectile faster and not turn the barrel into a pipe bomb.
For the most part many guns have already hit their practical limit in this regard.
There is a good reason why battleships fell out of favor other than the US being the only blue water navy after WW2.
Those big guns are a bitch they are slow to reload and you can’t be anywhere near the muzzle including good couple 100 yards of the bow of the ship when those fire.
Also powder takes up space and weight, takes time to reload and it can explode when ignited.
Having a gun with parts that don’t explode intrinsically when set on fire is a pretty big advantage for any combat vehicle or vessel, not needing to carry propellant for your rounds is also a huge plus because you can carry more rounds or other munitions and supplies.
The UK projected and fought a conventional blue-water naval war in the 80s.
Not sure if that's the reason, but it's probably a reason.
In the Battle of Jutland (The largest naval battle of WWI), the Royal Navy lost three capital ships (Two battlecruisers and a heavy cruiser) to single shells that struck their gun turrets, detonating their ammo magazines.
The gun turrets were, in theory, supposed to be sealed off, by blast doors, from their magazines - but in order to increase fire rates, it was standard practice to leave those doors open in combat.
The Germans, not being suicidal lunatics, kept equivalent blast doors closed, which is why none of their capital ships went up in flames, despite taking a lot more damage.
The Royal Navy had a long and well-established tradition of exceptionally well-drilled gun crews, probably going back almost 150 years from WWI through the Napoleonic era. It could be argued that their fast-firing and disciplined gunnery were decisive in more than one British victory.
HOWEVER....as you point out this led to objectively-suicidal practices....What's that term for when you've been doing a risky job for so long you don't realize how dangerous your habits are? Yeah Royal Navy gunnery is a textbook example of that.
Chemical propellant in first stage, electromagnetic acceleration in the second stage.
I'm curious if they want plasma projectiles explicitly, but I guess it's all kinetic at the end of the day.
The propellant for a large shell takes up more space than the shell it fires, it's tremendously dangerous to have around from both accidents and enemy impacts.
Compared with a rail gun which is room full of exquisitely machined inert metal blocks.
Cost - Those inert machined metal blocks are cheap in a military if not absolute sense.