It's just an older and not as well supported format compared to several others. In particular it is a Windows-centric format as it was developed by Microsoft back in the 80s and, unlike TIFF - which is a similarly old format - BMP remained largely Windows-specific. It is also comparatively more limited than TIFF (and other formats). It doesn't support embedded alpha channels or more than 8 bits per channel, for example, while TIFF supports both of these things and much more. TIFF is much more widely supported than BMP, particularly on non-Windows platforms. Basically, although BMP is "OK", there's just no good reason to use it anymore, it offers no real advantages over other formats and so has not remained very popular.
- Oshyan