A lot of it was a metacentric height issue. There were more than a few predreadnoughts (granted, particularly in France and Russia, but elsewhere as well) that had absolutely awful balance issues due to too much weight being up-high. So by not armoring the gun mounts, you both eliminated an enormous amount of topside weight, and didn't REALLY affect the ship's durability- a hit on that mount probably was going to disable it anyway, armored or not, so why bother? (This obviously was a flawed way of looking at things, which is why it didn't continue past this point... one suspects the gun crews weren't asked for their opinions either.)
It's fascinating to look at the predreadnought era. As Weirdo pointed out, this is 20 years prior to Dreadnought, but things advanced amazingly fast at this point- and not from one direction, but many. Remember, Dreadnought's biggest advancement wasn't the all-big-gun concept (that was being attempted at the same time in the U.S. and Japan, remember), it's that she had turbine engines. The guns were almost an afterthought when you really look at the design. Between that point and the above, there's a whole lot of really interesting and unique ideas as a result of many advances all made around the same time- weapon technology, engine advancements, shipbuilding techniques, etc.
The results were ships that varied wildly, with different navies having what looked to them like great ideas that often just didn't really pan out, but had to be tested to find that out. A good example is the American attempts to mount their intermediate batteries atop the main batteries, which seemed wise because it meant less big holes in the deck (thus a stronger hull), less magazines to hit, and less chance of a hit knocking out a chunk of the ship's main firepower. Of course, metacentric height was a big problem as a result, and a lucky hit could disable a HUGE chunk of the ship's weaponry- and as a result, the American Navy discontinued that idea after a couple of classes. No one else even tried it though- it remained a unique idea there. Tumblehome hulls are another one that seemed like a great idea to the Russians in particular, that didn't really pan out as well as they wanted it to.
It's a fascinating era to look through and see how different navies approached the new technology and tactics being forced down their throats faster than they could keep up- and how it set up the following dreadnought race, eliminating bad ideas along the way so that by the time Dreadnought raised her flag, most of the silliness had been worked out of designers' systems in favor of simple efficiency.