I used to buy as a dealer from Sudco, they've been around a long time.  I suspect the large amount of counterfeit Keihin and Mikuni carbs now for sale online, especially with the fake "Sudco" stickers on them, didn't help matters.  Having the OEMs reducing the number of bikes sold with carbs has probably also worked to reduce Sudco's business.

Another factor might be the age of the owners.  I'm seeing a number of businesses (like AirTech Streamlining) closing because the owners are retiring.  And with AirTech, which has a huge, make that HUGE, amount of molds for rare fairings/seats/fenders etc, if no one steps forward to buy the inventory to continue the business, then everything gets scrapped.

I have been a long term Sudco customer , feeds my addiction to Mikuni
carbs. Sudco stickers all over the tool box. What a shame ! Sudco manuals in the stack with all my other manuals/bibles.

Paul, Sudco would do retail sales as well as wholesale.  But I think they've got real stock in a real warehouse which is a cost a lot of the mail-order discounters don't have to deal with.  There's been so much consolidation in brick and mortar shops over the last 20 years.  It must be really disheartening to have someone come in, try on different helmets/gloves and get advice on tires and things and then announce "thanks, now I know what to order from the mail-order place" as they walk out.  A lot of the small and mid-size shops weren't making a lot of money to begin with, and had to keep some product in stock instead of having everything drop-shipped from a distributor.

On the other hand, I've gone into local shops expecting to be able to get a carb jet or something simple/common like that and been told "we can order that".  I worked in a Suzuki/Triumph/BMW/BSA shop when at university and we kept a lot of parts on hand, but that doesn't seem to be as common as it used to be.