I think a big reason is to cut down on attempted impersonation. A capital "i" looks very much like a lower case "L" for example.

In the few years I've been involved in altcoins I've seen countless scam attempts by hijacking a username and impersonating them by making very small changes such as that. Very rarely do users catch these slight changes and before they know it they send BTC to a fake exchange owner for a falsified market pump and promise of profit, never to be seen again.