Bans are permanent now because back when they were temporary people would just cheat the moment they get unbanned.

Valve started working on a "long-term solution" for cheating in 2001. VAC's initial release was with Counter-Strike in 2002. During this initial release, the system only banned players for 24 hours. The duration of the ban was increased over time; players were banned for 1 year and 5 years, until VAC2 was released in 2005, when any new bans became permanent.

https://en.wikipedia.org/wiki/Valve_Anti-Cheat

Temporary VAC bans were tried. Users kept cheating.

Valve was lenient and gave people second chances. Valve stopped being lenient and cheaters were the reason.

Originally posted by TheRealSuperMushi:

I don't know why, can anyone please tell me why?

Could you explain why not? If someone cheats they ruin games for others and if their account was compromised and used to cheat then the account holder should have been more responsible with their account. In either scenario a ban is gladly welcomed by the rest of the community.