My guess would be that some don't feel comfortable working with 100mg nic. Some might prefer to mix with a pre-mixed base, some may feel they might not use it up fast enough, especially if they vape really low mg. Not everyone buys in bulk and stores in the freezer. Some have curious minds and youngsters in their home, so they prefer to buy lower mg, and a size they'll use up in a reasonable amount of time, locked away from curious younglings.
I do feel that 100mg is the most economical route, but I can understand why people don't. The first time I purchased nic, it was 48mg. I was comfortable with that when just starting out.