I think they use different size limits to maximize a given fishery (what's the age of the body, what's the forage, what other fish make up the fishery, etc...). I read an article where the biggest challenge on determining what to use for a size or slot limit is knowing whether people will actually harvest. With a slot, they definitely want you to take the under fish and are ok with you taking the over. So, the ineffectiveness of a slot limit on a lake could be that no one is harvesting according to the slot.

As far as a trophy fishery, I think most of that has to do with the age of the lake and fertility. Briery Creek in Virginia is relatively young. It's only been around since 1986 and they've produced multiple fish over 13 lbs. There are trees, stumps, and grass everywhere. Dale Hollow was the same for years. It had stumps, willow clumps, and grass everywhere. Now, not so much. If you read Billy Westmorland's book, a good stringer had multiple fish over 6 lbs and a big fish was 8lbs or more. Now you feel really good to get a 5 or a 6. Age does take it's toll on us all.