Only the oldest games—Traveller, D&D—get away with the sloppy design of having rules that are kinda generic but really are spiked with loads of setting assumptions that you’d have to go clean out to really use them as a generic ruleset.
I think one of the real stregths of classic Traveller (following the lead of D&D) is that it has an implied setting. Yet, it doesn’t detail the setting. The setting is only implied.
There’s a lot to be said for the implied setting approach. It is quick to get into. The implied setting has already filled in enough blanks to get things rolling. Not too many, however, that would slow down getting started. It still leaves a fair amount of room for the group to really make the setting their own.
Classic Traveller (perhaps even more than D&D) also provided tools to aid in fleshing out the group’s unique version of the implicit setting. Those tools being presented as random generators gave us the flexibility to either randomly generate things or just make choices.
I’m glad there are games that strive to be more generic. I’m glad that there are games that are strongly tied to a specific setting. I enjoy games from both of those approaches. I’m very glad, however, that there are still games—like Mongoose’s incarnation of Traveller—that take a more moderate approach.
Look, no fault of Traveller's initial design, it's fine for the 1970s when people didn't know any better.
This is not sloppy. This is not ignorance. This is a very strong, middle-of-the-road approach. Moreover, I think such a middle-of-the-road approach is especially suited to newcomers to the hobby.
On mxyzplk’s criticism that Traveller never presents a main Imperium setting book: I don’t really know any of the editions except classic well enough to confirm or deny that charge. I can think of some arguments against it. Yet, I think it has merit. Even with such a book, however, I think having the implied setting in the main rules fits what Traveller was meant to be.