Dried bottom on a pork butt

I know when I've done big meats in my uds with no diffuser the bottom gets pretty crispy. Maybe turn it over halfway through the unwrapped time? I always use a diffuser on long cooks for just that reason. That's a long time for one side of the meat to be getting a direct suntan.

I can't count the number of butts I cooked when I had my WSM's. 90%+ of them I cooked with no diffuser nor water pan. I have never had one even remotely get crispy on the bottom. I do trim all the fat caps down to 1/4" or less and cook them fat cap down.

A few things come to mind here about your issue. Are you sure your grate temp was 250? WSM's are notorious for having grossly inaccurate thermometers. Did you cook it on the top rack? If you cooked it on the bottom rack it would be too close to the fire. Especially with no fat trim.

I'm really thinking that your problem is mostly caused by not trimming any fat and cooking directly over the fire. A little fat dripping on a fire is a good thing because it burns off and gives great flavor. A lot of fat dripping will cause a grease fire.

Don't own a WSM- but have cooked a ton on it's redneck cousin- the UDS. No pan, no diffuser. If going naked and not giving it flip or two- I have gotten the bottom "a little more than done"... but it never went a full inch into the good meat.
My more usual is to give it about 4 hours on the grate. Then into a SS half pan (love those) on a footed rack- foil it over and let it roll.

NO water pan or just no water in the pan but the pan was in?

No water pan at all may have allowed the WSM to draft upwards better and the cap burned up just because of the convection. The pan changes the way that thing cooks in more ways than one.