It'd probably be easier for Mevans to add these features into the mod than for him to update the entire thing to a new version.
For instance:
1.8: Literally nothing that concerns Middle-Earth.
1.9: Considering how pretty much everyone hates the 1.9 combat system, I think it's best to leave it out. Besides, the LoTR mod already has its own combat system, and it'd be easier to Mevans to simply improve that one, with the added benefit of originality.
1.10: Polar bears are the only thing worth adding from this update, and I'm sure the mod team could produce their own version with a better model/texture quite easily.
1.11: The /locate command would certainly work well, but let's not forget how many travelling conveniences the LoTR mod has already added (waypoints, an on-screen compass with coords, etc.)
1.13: I'd rather see the LoTR mod's coral blocks simply get a better generation pattern (generating as boulder-like structures in shallow tropical waters instead of scattered blocks at the bottom of the ocean). The Vanilla coral blocks look way too unnatural and flashy. As for the other stuff, like fish and underwater ruins, the mod team can add their own versions of those.
1.14: Again, it'd be easier for the mod team to improve the current trading system than to update the entire mod. And the same applies to raids/invasions. In addition, the fact that the mod's invasions are unpredictable makes them more of a challenge and much less exploitable. Raids, on the other hand, are nothing more than a loot farm once you've got a good wall up.
The only benefit for updating the mod would be an increased audience in my opinion.