It’s the same reason that they don’t serve McRibs all year round.
No follow-up questions.
There used to be a chain in LA in the 60’s-70’s called Pup ‘n’ Taco that sold hotdogs and tacos. They were cheap and good, and you could get a whole box-full for a couple of bucks.
Especially not Schnitzel made from the natives of Vienna. 
They’ve got them down in Texas too. Pretty good dogs there. When I see one, I like to stop in for a chili dog or two.
Anyhow, it seems that while there ARE fast food joints that serve hot dogs (DQ, Sonic, and DW have been mentioned), burgers are just the more mainstream drive-in fare. You might as well ask why major fast food joints don’t serve tacos and burritos (except for the ones that do: Taco Bell, Taco Bueno, and Whataburger all come to mind).
Buffalo has Ted’s, and a bunch of smaller independent hot dog stands. Traditionally, hot dog stands in the Buffalo area are owned by Greeks. Like Chicago, it’s a city with high “hot dog awareness”. Some cities seem to be crazier for hot dogs than others, I guess.
And used to have Pat’s until they up and moved. Pat’s was the best.
Sonic recently added several varieties of hot dog to their menu. They’ve sold cheese coneys for over 40 years.
The name was chosen after they found out that most Californians of the time thought “schnitzel” was German for “sausage”. Never let reality get in the way of careful market research.
Also, the company name started with “Der”, the German masculine version of English “the”. Never mind that in German, Weinerschnitzel is a neuter noun & “Das Weinerschnitzel” would have been correct. The corporate marketing folks thought only “der” was instantly recognized as connoting “German” by the locals, most of whose knowledge of all things German was based on Hogan’s Heroes, then prime time TV fare.
Eventually the corporate name was changed to just “Weinerschnitzel”, but that was more a response to being bought & sold a couple times than any desire to correct a grammatical gaffe.
And to top it off, there was exactly zero about their menu which was German. It was just a hot dog stand. But hot dog == Sausage == German was the marketing gimmick & so it was played that way.
B…b…but I thought whale sperm was a traditional German dish 
And they still use that perception in their ads, even though they’ve dropped the “Der” from the name. Everything they sell is “Der-licious!”
They actually do a not-bad line of dogs. The bacon-cheese dog is a favorite of mine.
They’re not great; but sometimes, as with Jack In The Box tacos, you just have to have a couple Wienerschnitzel chili-cheese dogs. When you want 'em, there’s nothing better. ‘Real’ chili dogs don’t hold a candle to them. Unfortunately, the one Wienerschnitzel I knew of up here is closed.
I never went to a Pup’N’Taco, but I remember their ads. Dad and I were watching TV once, and I was flipping channels. I went from a religious station to a Pup’N’Taco commercial. The dialog came out as:
And the banquet was laid out on the table.
Pup’N’Taco! Pup’N’Taco! Pup’N’Taco!
That’s good to know. Are they grilled? I could go to Dairy Queen, which is a fast-food burger joint that sells hot dogs. I could stop into a dive bar called Shorty’s in Belltown. I could go to Arco or 7-11. But it’s bloody difficult finding a grilled hot dog instead of one of those ‘roller dogs’.
They had them in Utah, too. They annoyed my very German professor, since they did not serve Wienerschnitzel (basically breaded veal cutlets, although German food aficianados will be coming after me for that characterization). It’s obviously a name come up with Americans for most other Americans, who neither know nor care what the word really means.
Stewart’s, on the East Coast, specialized in the 1950s and 1960s Hot Dogs and Root Beer and Fries, served in trays at your car. They’ve decreased in number, but they’re still around.
Amusing note from the Wikipedia page on Wienerschmnitzel restaurants:
“Wienerschnitzel ist PEOPLE!!!”
Hot dogs are sometimes called wieners, and Wienerschnitzel is a German word that sounds better for a restaurant than ‘Wiener-worst’.
Could you supersize my McWiener?
(and add extra special wiener sauce)
The real question hasn’t been asked:
Given how successful the hamburger chains have been, why would anyone selling hot dogs not try to sell hamburgers?! 
When I was a little kid (5 or 6 years old, around 1970), my family would occasionally go to Burger King, where I’d always get a hot dog (since, at that formative age, I still didn’t like hamburgers). One day, we went there, and I was crestfallen when we found out that they no longer had hot dogs. I grudgingly had a hamburger instead…and a love affair was born. 
I wonder if part of the reason for lack of hot dogs at many national fast-food chains is regional tastes in hot dogs (especially the toppings). What you get on a Nathan’s hot dog in New York is very different from what you’d get on a Portillo’s hot dog in Chicago.
You sure you’re not thinking of Das Schnitzel Haus? They actually serve schnitzel there and not hot dogs 
Weinerschnitzel has burgers too.
Several Weinerschnitzels in town here; we refer to it as “Weenie World”. My wife loves the chili cheese dogs and chili cheese fries; the kiddo like corn dogs or occasionally a regular hot dog. Me, I like the all-beef stadium dogs. They also have “pretzel buns”, which I haven’t tried but I hear are delicious. I’m pretty sure they sell burgers too, but hot dogs are their main draw.
They’re also allied with Tastee-Freez, which means their soft-serve ice cream cones are better than Dairy Queen’s, IMHO.