Because he has proven he cannot be trusted.
He has already endangered numerous lives, including his own, while driving drunk. There are numerous extremely bloody cases of DUI, including one right in my home town where said driver went flying through the windshield and ended up mangled so horribly they had to cover him with a blanket of some sort, and one of the cops was vomiting.
A drunk person who has established that with a car they are a danger to themselves and others cannot be allowed to drink further, because soon enough, they will find other ways to be a danger to themselves and others. As we see on America's Dumbest "X Y or Z", cars are entirely unnecessary for drunks to endanger themselves and others. There are many other ways for that to happen.
Thusly, a person who has endangered themselves and others while on a substance cannot be allowed to partake in said substance, given they have proved to the law that they are unable to be trusted while on it. I am not willing to allow someone the liberty of using a substance if they have proven already that they are unstable and unable to be trusted while on it, and thus a possible danger to the community around them.
Pretty much this. Not unlike banning someone who's been committed of a firearms-related crime from being allowed to own guns.
That makes sense, though I still think just banning them from driving would suffice.
There's a very, very small amount of places in the world where that wouldn't be essentially sentencing the person to homelessness and poverty. Reliable, effective, easily accessible mass transit with a decent amount of stops is actually the exception in the world rather than the rule, and is pretty much entirely exclusive to large urban environments like Los Angeles, Tokyo, and New York City. In a lot of areas, especially the entire state of Florida, having a vehicle is
vital to being able to get anywhere in a respectable amount of time.
To give a personal example, I live in a middle-class area in the Orlando urban sprawl. The bus system is infamously slow, to the point where a 15-20 minute car trip can take 2 hours by bus and include a transfer halfway through. Even if I took the bus, the nearest station (as well as the nearest gas station/convenience store) is a mile away from my house down a featureless road, nothing but suburb entrances, trees, and suburb walls lining the sidewalk. Many suburbs sprawl out so much that you need to travel
4 miles simply to get to a commercial zone.
And this isn't some random podunk town off in Idaho, and it's not a rural area. It's just regular urban sprawl and suburbs on the edge of a major city. Even within the "official" Orlando area, mass transit is very small and only efficient in the immediate downtown area. Florida was simply designed for cars, with walking and bicycles being an alternative to owning a car or a method for underage children to get around.
Banning someone from driving for a DUI would make their lives almost unreasonably difficult, and at a minimum would likely cause them to lose their job due to difficulties showing up to work on time.