Good Evening, Pamela,
As many above have noted, the biggest reason is the improved print quality from the larger negative.
Actually, I started with a Ricohflex TLR when I was doing yearbook photography in high school. Unfortunately, I had no darkroom then and did no printing. After college, I bought a new 35mm SLR (that style was "in" then) and used it happily until I set up a darkroom several years later. I was greatly disappointed by the print quality from 35mm negatives; of course, Tri-X was big then, but so was the grain it produced in prints. Plus-X was bearable, but I never could come to like it. (Where was T-Max when I needed it?)
In a photo store one day, I bought, on impluse, an old Ansco Speedex folding camera. After printing the first 120 negatives, I knew that MF was for me. I went on to other TLR's, a Mamiya Super 23, a miniature Speed Graphic, the Koni-Omega system I still use, and, recently a Fuji 6 x 7 rangefinder. Overall, my preference in MF is 6 x 7 or larger. Naturally, all this soon led also to 4 x 5 . . .
It all comes down to print quality. Getting a good print from 35mm is work; the larger negatives make everything a lot easier.
Konical