A brief primer on commercial realities:-
It costs Sky/NowTV to carry any programmes except those it makes itself, and The Flash isn’t theirs.
First run rights typically carry the right to catchup availability for only 30 days after first broadcast, and after that, other broadcasters are clamouring for second and subsequent rights.
Older seasons are cheaper, and can be run for longer, as there is less demand for them; the ‘box sets’. Though even these come and go, as while they are cheaper to obtain, they still aren’t free.
NowTV have to balance what they spend with the best ‘bang per buck’ they can give their customers, and the customers will pay for.
So not selfish at all; nobody gets a new Season first run indefinitely, not at least for any cost that would be merited. And Season 5 will be back around as part of a box set, when available, and when the price is right.