For the same reason Guinness is available everywhere – it is actually brewed in Japan by Kirin. I could also speculate that its bland taste suits those who are not used to real ale, but not sure who actually drinks Bass, which might explain why it is available so cheaply in supermarkets (about 225 yen for a small bottle).
Something I noticed which could also do with explaining is how both Guinness and Bass menu chalkboards appeared outside restaurants long before the beers became widely available – and still many of those places don’t have those beers in them. A clever marketing trick by Kirin, or did they all just like the retro look?
Should you be interested in drinking some decent British (or British-style) beer in Japan rather than the bland travesty that Bass has become, that is a regular topic over on my Tips for Brits in Tokyo blog: