The main reason why bilaterals are generally considered a "luxury"...
There is a cost associated with everything. Since most CIs are funded via societal efforts (i.e., insurance, gov't funding, private foundations and etc.), it becomes an issue in how best provide maximum "benefits" to the population at large.
It has been shown that one CI provides the best "bang for the buck" and thus such "generosity" focuses on at least providing that option. In cases where it is deemed necessary (i.e., deaf blind and other scenarios), then bilaterals make sense. ***Remember, having one good ear is sufficient for most people to get along as productive members of society.
Now it seems that in some instances (at least here in the US), insurance companies have started to cover bilaterals. Whether that will become a general trend, that remains to be seen. I guess the big question for insurance companies is whether or not this would be a good way to allocate scarce resources (i.e., limited funds). It all comes down to money as always.