The .38 Super is a higher-pressure round than the .45ACP, so in gun-games such as IPSC where compensators are common, the extra pressure gives the comp more to work with - it dampens the muzzle-flip better. It's still popular among the older guys who originally used it back when there wasn't the depth of caliber choices we have today such as 9x21, 9x23, 40S&W, 10mm, etc.
Also, some countries don't allow their citizens to use the same calibers as the military, so the Super is popular there for civilian use (IIRC, countries such as Mexico and the Phillipines).