Because if you want people to buy EVs, higher taxes are a disincentive.
If you put people in a position where an F150 costs as much to own as a Model 3, they will almost always choose the F150. Is that really what you want?
You can have regulations either completely banning fossil-powered vehicles, or keep raising carbon pricing to encourage the move to EVs. But if those EVs cause road damage, people should pay for that damage.
All that is being done in both cases is forcing people to pay the true cost of these things (pollution, road damage), and bringing to light what have been externalities until this point. Let The Market™ decide.
If people want to reduce pollution, and not have to pay for the damage their vehicles cause, then perhaps they should use lighter vehicles like (e-)bikes or (e-)scooters / -motorcycles. Or transit.
And it doesn't have to be a legal ban: just bring in the cost of externalities. You can drive an Old School Hummer all you want: just be willing to pay for the pollution you are causing.
You can drive the New School EV Hummer all you want: just be willing to pay for the road damage you are causing.
I think everyone knows the deadlines that are set for phasing out fossil fuel vehicles are not going to actually be enforced. They are nothing but virtue signalling from politicians.
If those things are actually enforced, you can expect a blowback that will make the election of Trump look like a minor detail compared to the insane populists that an angry population will put in power.
Manufacturers are planning to phase out fuel-powered cars (perhaps completely, but at least in some countries/regions), so governments may not have to do much enforcement themselves:
And? If they're heavier they cause more damage to roads:
* https://en.wikipedia.org/wiki/Fourth_power_law
So why shouldn't they be charged proportionally to the wear they cause?