As many players have noticed, there is a catch-22 in S1 in that you can't build a terminal unless you have a hub, but you can't declare a hub unless you have 5 gates. At airports with no gate availability, this has prevented airlines from establishing a foothold.
I have now lowered the minimum gate count for declaring a hub from from 5 to 1, so this should no longer be an issue. Note that this change applies in S1 only.