Hi @Anonymous User,
Previous seasons for many TV series' are usually made available on the NOW TV service for a few weeks before the new series begins. The length of time they remain is down to pre-arranged agreements with their copyright holders. Some stay longer than others.
As with anything, it all comes down to money. Which is why the 'catch-up' episodes from the newest seasons (usually) expire 30 days after their original 'live' broadcast date, much like they do with BBC iPlayer.
Cheers
