SPECULATION I will finish what you have begun...

What if Kylo Ren and the Knights of Ren's objective (under the influence of Snoke) is to finish what was believed to be Darth Vader's final mission:

To turn Luke Skywalker to the dark side of the Force.

Maybe that could all be the reason why Luke is hiding.

He's Vader's son, possibly the most powerful Force user in the galaxy.

Luke left Leia and his friends behind at the first place because he feared (after what happened on the second Death Star) that his weakness is him caring for his friends and the Rebellion... and, of course, Leia.

So he go in exile to start his own Academy? or live a quiet life somewhere.

The Knights or Ren track him, attack his academy or settlement, trying to push him to the limit... instead, he just goes into exile, maybe sending back R2 to transmit a final farewell to Leia.

Or maybe R2 plays a bigger role in protecting young Rey if she was indeed one of Luke's pupil.