Simple Economics;
A 200 rd. 9mm Winchester Range Pack is $47.77
A 200 rd. 40S&W Winchester Range Pack is $62.63
I can pay for my range tine with the cost difference
The .40cal also filled a particular role at the time of the AWB. With magazines limited to 10rds no matter which caliber you chose, 9mm was losing a PILE of ammo and most .40's were only losing 1 or 2 rounds. And double stack .45's were just not at all popular.
For a period of time, it seemed to make a lot of sense to carry ten .40's rather than 10 rounds of 9mm. And the 9mm had a reputation for not being much a cartridge -- but at least you had a huge pile of them. (until you didn't anymore)