I like fish and chicken. But I do not eat it breaded or fried; I only eat it grilled or perhaps blackened. I think it tastes better, and it’s certainly healthier. So why is it so many casual & “lower-tier” restaurants (including bar & grill places) primarily offer just breaded or fried dishes? Is it because most people prefer them fried? Or is it because fried is cheaper than grilled?
In a limited space kitchen, fried is easier than broiled/grilled. You already have a fryer set up for fries/mushrooms. A grill/broiler is another footprint in the kitchen. Also, for broiled/grilled you need refrigerator space for trays of fish/chicken. Fried fish/chicken can be cooked from frozen, which is far simpler for the cooks.
Fried allows quick preparation. Some people like fried food and frying is a messy process at home so those who like fried food may prefer to go out for it.
It can also be ordered like that, frozen and pre-breaded, from suppliers. If we’re talking lower tier restaurants, I’d assume the majority aren’t doing a flour, egg wash, breading process on protein from scratch. Just knife open the plastic and drop the product in the oil. Faster, easier, and much cheaper.
If I am buying fish for a restaurant, breaded, frozen fish is cheap and easy. If I’m buying fresh fish for grilling/broiling, then I’m buying large pieces and having my kitchen people cut it down to serving sizes and arranging trays.
Fresh fish goes bad at some point. Frozen, not so much.
This is somewhat me. Fried fish and chicken is fantastic and I’ll take it over any other prep if I’m not considering my health and calorie intake. I also don’t prefer making them at home. My kids will not eat a regular fried or grilled pork chop, but panko crust it and they’re all in! I think a lot is we like fried foods.
Another part of the equation, perhaps, is that fried foods tend to be saltier (especially when accompanied by ketchup or another salty dip or sauce), and salty foods promote thirst, which promotes the purchase of beverages, which are high margin products for restaurants. At the lower tier, that’s likely an even bigger contributor to profitability.
(cf. the famous justification for bars giving you a little dish of pretzels or peanuts at no charge.)
Breading and batter cost less than meat. The volume and weight of the fried product contains more low cost ingredients than the meat alone, plus more calories.
In addition, a lot of restaurants need french fries on the menu. Fryolaters take up space, have to be kept hot, and need cleaning and maintenance. That creates a minimum necessary cost for cooking fries. Using the fryers for other dishes adds only a small incremental cost.
Quality is also a factor. Bog standard white fish fillet or lean chicken breast is not going to be particularly tasty just grilled. But will be perfectly edible deep fried. On the other hand if you have a beautiful fresh piece of fish it would be criminal (or should be 
) to deep fry it
I am with the OP on this generally, deep frying is rarely the best way to cook anything, with the possible exception of chicken (and only then if it’s done well) I will take a good roast over frying any day
Not if the customers have ordered soft drinks, as unlimited refills are standard at lower-tier restaurants.
As for sodium content, I’d expect it to vary depending on the restaurant’s source and preparation, but oddly enough this source lists a much lower sodium content for battered fried fish fillet compared to grilled (fried floured or breaded fish typically has a higher sodium content than grilled, though not tremendously so). 
That is odd. There is often no difference between freshly battered and breaded fried fish except for some more water to make a batter. The mixes are usually not heavily seasoned and salt free versions aren’t uncommon these days. Bread crumb coatings are more often found on frozen fish products and all frozen ready to cook products are more likely to contain additional salt. I think frozen products have thrown off the sodium numbers for all the cooking variations of fish in that chart.
I think the question has been answered, but I’ll add that I’m one of the people who eats fried food only at restaurants. I generally prefer grilled fish to fried fish, but i love a nice piece of fried chicken. And having tried frying a few items at home, I’m not doing that again. You end up with a bottle of slightly used oil that can’t really be used for anything except more frying. And i don’t want to eat fries food all the time. (I don’t love it that much, it’s messy to cook, and it’s not terribly healthy.) So i agree with whoever speculated there might be some extra market for fried food at restaurants.
Yeah. 100% comes frozen, pre-breaded.
They couldn’t sell a fish sandwich any other way at a low tier restaurant.
Gotta pay bigger bucks for fresh fish. Prepared any way you choose.
Maybe in it’s a problem in the midwest but some of us live near the big fish farm called the ocean. There are still plenty of places around here to pick up fresh fish and chips and other fried seafood. Sadly it has become more expensive but it’s not entirely gone.
I used to work with a guy who had previously been a health inspector. He referred to breaded and battered foods as camouflaged. So KFC became Kentucky Camouflaged Chicken.
OP was talking about lower priced joints.
I’m in the South. Trust me. I have all the fresh fish I’ve ever wanted. In my freezer. Wild caught.
I do not eat fish in restaurants. Ever.
These are lower priced joints. Not many dedicated small seafood restaurants anymore but it’s quite common for small pizza/deli restaurants to offer fish&chips and fried seafood. If was far more common around here before all the old Catholics died off. It’s more expensive compared to other types of low end/fast fool now because the price of seafood has climbed so high but burgers from McDonalds cost a lot more now too.