Junos OS

https://kb.juniper.net/InfoCenter/index?page=content&id=KB21476&actp=METADATA

For what reason is it not updated?

JTAC recommended versions change when the situation warrants, not based on an arbitrary schedule. I personally consider these recommendations to be the conservative choice and make a good jumping off point to pick the release (you'll notice different MX configurations have different releases, for example). You can always discuss the pros/cons of specific releases with your local account team or partner. 

In the direction of what version should I watch?

For MX480s(bgp border + stateless firewall + bgp flowspec + jflow)  ~300Gbps per router

Because 3.13R9 end-of-engineering ...

The MX series has a dual recommendation on the JTAC kb above:

Junos 13.3R9/14.1R7

This gives both the current full support version and the older stable version option.

A note to remember is that JTAC does not automatically shut down tickets on EOL versions of Junos on supported devices.  They will research your issue and tell you if an upgrade is required because of a known bug (PR in Junos speak, problem report).  But if your issue is not related to a known bug and you have a support contract you can proceed.

Naturally, at end of engineering no new bugs are fixed going forward so there is a risk there.

But JTAC recognizes that upgrade verification can be a slow process and does not force your hand without need.

So you do have the option of continuing with older versions if your systems are not fully tested and ready for upgrade.