It is simply a convention because 30s was deemed a reasonable limit. Nikon, Pentax and Sony all use 30s but Olympus uses 60s. Panasonic uses 60s on most camera but up to 250s.
As you can tell by the presence of Bulb mode, most cameras can do more. Olympus limits theirs to 30 minutes to avoid the sensor over-heating or building up too much noise. Other brands can expose for hours.
Note that the available range of shutter-speeds depends on the mode. In Manual mode, you can choose any. However, in modes which rely on metering (S, T, A, P, TAv, etc), you often have a shorter limit (1s to 4s is common). An even more bizarre variation are those DSLRs which have a different limit in Live-View than otherwise.
For automatic metering, the issue is sensitivity of the metering system. At some point, there is not enough light to meter and so the camera cannot know if it needs a 30s, 45s or 1200s exposure.
There may be a practical reason too - but this is speculation - and that is in case the exposure goes to 30s (or more, depending on your camera), you do not have to wait so long for the next shot since most cameras do not let you interrupt an exposure.