I suspect it's one of those things where there is no single, obvious reason why they were such a success but it was more a balance of probably quite numerous different factors that between them appealed to a broad cross-section of gamers; and managed to get it right in that the stuff that appealed to people generally comprised a more significant factor than the bits that put people off.
It makes me think of a sort antithesis of the infamous marketing of the Vauxhall Vectra, a boring '90s car that nobody remembers for good reason: they spent so much time thinking of ways in which it wouldn't offend anybody that its real offensiveness is that it had no personality. I'd hazard a guess that the appeal of DOS is that it wasn't afraid to have its own personality but had enough discretion to keep the various expressions thereof at a sensible level. And sure, it really irritated a lot of the purists who thought it didn't go far enough with one thing and another, but they seemed to get it right.
Personally I'd like to see the classic Larian humour ramped up again in future releases: it was dialled down quite a lot in DOS2 to cater for the "i r srs gmr" types and I didn't care for that sort of thing, but I guess the point is they kept it within reasonable limits. So I may have lost some of the more ludicrous bits of silliness and just random stuff like doing a handstand as the final part of climbing a ladder, but we still got the outrageous Brummie accented knight referring to us as "yow" as often as possible and the sneaking barrels, the latter of which really upset some people. But not enough to not play.
