Here's the thing...
...and this is my opinion only. For some alcoholics it isn't-- they are the ones that stay and fight. No matter how many times they relapse they pick themselves up and keep fighting for sobriety and recovery. Some succeed, some don't, but they fight to the bitter end.
For others it is easy and the reason is that they never loved at all. You were simply the current enabler for whom they had to maintain the illusion of loving you in order to keep you enabling. And, when you stopped enabling, you no longer had a purpose and had to be replaced. For that type of alcoholic you are no different than the tires on a car. When they wear out your replace them, and the only angst around it is that it's kind of a pain in the ass to replace them and you may need to spend a little dough to buy the new ones (unless you can find somebody else to pay for them, and that's what they specialize in).
It's as brutal, uncaring, and as simple as that.
My two cents.
Cyranoak