It was cancelled due to the lack of support of the show. The second season is still a "complete" season, it just wraps things up pretty quickly.
If you're still interested in what happens after season two there is apparently a comic that came out to continue the story where it was left off. (Not sure if its fan made or fan supported or made by the creators.)
It was meant to be cancelled after season 1 but after a large amount of people were upset they gave season two a try to revive it and it apparently failed.
Comment from one of my questions about Jericho:
The reason for the rushed ending was that the show was actually
cancelled after the first season due to poor ratings. However, it was
a cult hit and a number of fans protested the network to get another
season commissioned. This happened; it got another 7 episodes. But it
got cancelled again after the second season, hence why it all got
wrapped up prematurely and so many corners were cut.
– Craig Russell
Another comment from my question:
Not sure, however, you may be interested in checking out the Jericho
comic. It picks up as a comic book version of season 3. So, if you're
a Jericho fan and interested in finding out more about what happens,
I'd look into this. As a note, although I do have the comic, I haven't
read it yet so I honestly don't know how good it is!