This is more of a practical consideration then anything else. While certainly not a hardware restriction, it is rare for a typical consumer to require more than three frames of AEB or a difference of more than +/- 3 EV between bracketed images. In fact, Nikon D3x00
cameras since the D3100 don't even support automatic exposure bracketing. Less experienced DSLR users are likely to get confused by a feature that can cause unexpected exposure behavior if used incorrectly. See Why are my Nikon D5100 M Mode (Manual) settings reset for every shot whilst the camera pointing to the same object?Why are my Nikon D5100 M Mode (Manual) settings reset for every shot whilst the camera pointing to the same object? for an example of how AEB can end up frustrating users who don't understand this feature.
Exposure bracketing has historically been used to provide a safety net for difficult lighting conditions, especially with film, but this feature is rarely used with digital cameras and is often unnecessary unless you're doing HDR. The camera I use, the Pentax K-5, allows up to five frames of exposure bracketing with each exposure separated by up to 2 EV, but I have never had to bracket more than three frames at +/-1 EV.