Well, it may be true that BC doesn't have an engineering program, but that's not why the institution doesn't call itself a university.
In truth, BC is classified by the Carnegie Classification system as a "Research University: High", meaning they're a university that (a) grants doctorates, (b) conducts research at (c) a high level of output. For what it's worth, by comparison, UConn is classified with about 100 others at the "highest" level, "Research University: Very High" (BC's tier of "RU/H" is about 100 as well).
The reason BC doesn't call themselves a university is primarily (a) name recognition, (b) the existence of a Boston University, (c) marketing. Again, for what it's worth, Dartmouth College is technically a university based on its doctoral and research output, and is classified in the "RU/VH" tier with UConn; they still call themselves a college primarily for branding reasons.