Why is fixed better on wet?

Fixed is safer in the winter as you dont need to rely on your front brake so much. Most accidents are caused by pulling too hard on the front brake on wet/icy roads, especially when cornering.

As you are able to regulate your speed nice and smoothly with your legs, the bike handles better

Off topic but,

My first bike was a beat up peugeot racer, the rear brake didn't work too well (not at all really), as a consequence I learnt to perfect braking with the front and not rely at all on the back brake at all.

Subsequently though I find it really hard to pull endos on my mountain bike, I always want to release the brake.

I find a back brake is only any good at controlling the speed going downhill (road) for obvious reasons.

Though I have found that if you are going down something steep (offroad) on a moutain bike using the rear to control your speed the back will lock up quite easily, so you end up feathering the front brake with your arse over the back wheel.

BTW I don't ride fixed (Yet).