Yes as Sven said one of the main reasons was for vinyl cutting. The more stereo information the wider the groove which in turn meant it had to be more shallow. This had the effect of a lower level cut and worse sound quality.
And also yes bass is omnidirectional so it doesnt make a lot of sense to have it "wide".
That being said we are talking about low frequencies here and many times basslines container higher frequencies as well, which would be suitable for stereo processing.
For PC users here's a cool freeware effect that will mono out your low freqs while retaining the higher ones in stereo:
http://www.otiumfx.com/basslane.php