You haven't been able to ban people who are votekicked for a long time, now, the reason is obvious, it was frequently abused. To the general noob population that plays dota, kicks and bans are all just weapons in their arsenal, if they can use them they will use them, and will have their epeen grow for doing so. Giving these people any real power is never a good idea.
There's 2 scenarios.
1. The player genuinely deserves a ban
2. The player does not and there's just an angry noob trying to 'get them'
90% of the time, it will be 2. The other 10% the chances of everyone or near everyone supporting the ban are slim, it would take 2-3 times before any chance of being passed. Now if this many chances are allowed, then in all cases of 2, the majority players can just constantly spam !votebans until everyone agrees, which is what happened with !votekick before they disabled banning from it.
Simply put, it's a bad idea. A very bad idea.