Why would you say the 1911 was no designed for 9mm? JMB did not do the design work, but someone obviously did somewhere along the way. The 9mm 1911 is relatively new (compared to the 45 ACP 1911, anyway -- still older than a Glock and othe modern plastic options), but 38 Super 1911s that don't do much ballistically than 9x19 doesn't do have been around for a very long time.
As for why buy one, why not? Not everyone feels underarmed with a 9mm pistol, and a single stack pistol tends to be more concealable for daily carry than a double stack, all other things being equal.