Once upon a time, the reasons were quite practical. Back when shutters were timed mechanically, the long shutter speeds involved running a mechanical timer -- i.e., the first shutter curtain would open, then the time would run, then the second shutter curtain would close. To let that timer run for a longer interval, you needed a larger mainspring that took more turns to wind. As such, the maximum shutter speed affected the overall size, cost, and usability of the camera.
Modern shutters are timed electronically, of course, but the longest shutter speed supported hasn't really been an issue/selling point for most, so it's remained pretty much the same for quite a long time.
From a practical perspective, 30s is also long enough that exposing by hand is almost certainly plenty accurate. For most people, doing a 1 second exposure by hand would be pretty challenging. Reactions faster than 1/10th of a second are quite rare, and noticeably slower than that are fairly common.
When you're talking about an exposure over 30 seconds, however, being off by even a second or two no longer makes much real difference. Just for example, let's assume you wanted something that was 1/3rd of a stop longer than 30 seconds -- about the smallest increment most cameras directly support anyway. Since one more stop would be a minute, 1/3rd more would be about 40 seconds. As long as you hit somewhere between 37 and 43 seconds (or so) your accuracy is probably close to as good as the camera will do anyway (and possibly more accurate than its fastest shutter speeds).
If you were starting with a base exposure of, say, one second, you'd need reactions on the order of an Olympic athlete's to even have a hope of getting that kind of accuracy and/or repeatability. Granted, most people could probably do all right at least than 30 seconds. They could probably remove the 30 second and 15 second spots without any real loss, and 8 seconds probably wouldn't be a big problem either. Much below that, however, would probably be getting a lot more difficult to justify.