They never standardized black and white photography development so it all has to be done by hand, which is more time consuming.
As for 35mm, yes it's true that it is cheaper, but I got into this for the larger negative, so this is a self imposed financial hit.
I think the reason why it is more expensive is just because it is a specialty service. Even in a large city, the amount of B&W film processed per day is not going to be very large, in 2022. On top of that, it's easy enough to process B&W yourself--it's not prohibitively expensive and can even be done in a small apartment without too much fuss.
(One reason that there are standards for B&W film processing is because it's necessary in order to have standards for film speed. The "ISO speed" of a film is tested by exposing the film to certain amounts of light, developing in a standardized way using a standardized developer, and then measuring some certain part of the resulting density curve.)
The 'default' process for processing colour film is C-41:
* https://en.wikipedia.org/wiki/C-41_process
You can get B&W film that uses the same process:
* https://www.ilfordphoto.com/xp2-super-35mm
One of the hard things about colour film processing in the past was that the chemicals had to be in a narrow temperature range: nowadays that is relatively easy to do by 'simply' keeping them in a warm bath kept in the correct range via sous vide equipment.
Generally if you're doing analog B&W photography at any non-small volumes, it's probably cheaper to get the gear to have your own darkroom and at least process the negatives yourself. Once you see the shots, you can then perhaps take them to a printer if you don't want to do that yourself.