I don't recall there being a minimum character count for SSR codes, but in legacy reservations systems (and the passenger name records (PNR) that contain these SSRs) there's for sure a max. Airline reservations systems never met a fixed-width format they didn't like, and the reason is that for a long, long time the unit of storage for a PNR was a block on disk and disk space was precious, so every few bits (literally bits, not bytes) counted. Those constraints have persisted through the ages.
Upthread someone wondered about disrupting this industry. It is expensive (there's a lot of moving parts) but it's basically engineering and integration, with the integration being the far harder part. For example, to participate in the airline networks (the networks that send messages to other airlines to handle cross-carrier booking, talking to GDSes, control departure, speak to DHS, etc) you need to understand the meaning of various airline protocol messages. These messages have unclear semantics; one airline may regard message A as meaning "create a ticket for this passenger" while another airline may regard the same message as "create a reservation for this passenger" which are two different things. The only way to know what these messages mean is to get on the phone with each airline and ask. The documentation is wrong and outdated; the meaning of those messages is the outcome they have to that specific airline and not anything else. Oh and the network connection you send these messages over is a VPN that you need to agree to setup with the carrier you want to talk to. Airline messaging is the reason most modern reservations system can only handle small airlines, since getting the semantics, routing, and other details of airline messaging requires too many humans and too many business relationships.
Additionally, switching reservations system can kill an airline. Most airlines operate with a day or less of cash reserves so if the switch to a new res systems stops operation of the airline for any amount of time you could kill the airline. There's no incentive big enough to switch to a new res system since the rewards are not immediately revenue-impacting.