arent suppressors illegal
Where you live, maybe. Not in America though.
Why can't intermediate cartridge rifles be suppressed also ?
Well you can put wheels on house, but that doesnt mean it will drive anywhere.
Intermediate rifle cartridges suffer from:
High bullet flight noise. They are supersonic, they make 135-140 db the instant a bullet leaves the barrel. That noise doesnt trace back to the shooter, fortunately, it follows the bullet. But it is noticeable that someone is shooting a rifle, if this is a concern for you. Hearing "crack crack crack" would get my attention, as well as the attention of any cop or NG in earshot of the bullet's path.
High report noise. This is addressed by the suppressor.
High gas system noise. The suppressor increases backpressure. For all guns this increases fouling but also noise if the gas system vents to atmosphere. This is like having a second 130-140db report that the suppressor doesnt affect. AR doesnt have this problem, but gets extra fouling and heat problems when suppressed. If your gun is prone to the "occaisional jam" it will become prone to a lot more of them when suppressed.
fairly high action cycling noise. Hammer fired semiauto = 120+ db of noise. I think that the striker fired guns get down to maybe low 110s for action noise, but this will only be relevant on a subsonic gun with no other noise.
Using subsonic pistol cartridges and blowback operation gets rid of bullet flight noise and gas system noise. Using a bolt action or a padded buffer and using a striker fired system can partially eliminate action noise as well. A mammoth suppressor brings overall barrel length up to 16 inches and gets rid of the report. Not only will no one know you are shooting, but whatever you are shooting at will be largely oblivious unless a bullet strikes near them.
Here is a chart of bullet noise to velocity. Above supersonic = loud.