Performance, mainly. An std::deque has all of the
functionality of std::vector, at least for normal use, but
indexing and iterating over it will typically be somewhat
slower; the same may hold for appending at the end, if you've
used reserve. And of course, std::vector is the default
container, using anything else will suggest to the reader that
you have special requirements.
std::vector also guarantees contiguity, so it (and only it)
can be used to interface legacy functions which require a T*
or a T const*.
I might add that the one time I actually had a performance
issue, and measured, std::vector was faster than std::deque,
despite the fact that I was regularly removing elements from
the front (using the container as a queue, pushing at the back,
and popping at the front). I don't know if that generalizes,
however; in my case, the queue was relatively short (never more
than about 15 elements, and usually many less), and the contents
were char, which is extremely cheap to copy. But in general,
I'd use std::vector even if I needed to remove elements from
the front, if only because of its better locality. I'd probably
only consider std::deque if I expected thousands of elements,
which were expensive to copy.
