They originally couldn't fully isolate just Lennon's vocals from the original recording, but technology has now advanced to where they can. That's the why it is released now.

Why not? It’ll give some people a bit of pleasure, it’s interesting to hear.

but seriously, why?

It's a pretty cool and special release in my opinion, no need to be a hater. Literally all of the Beatles were included on the track as each of them touched it in some way over the last 30 years: John's now isolated vocals from his demo tape was the starting point, George's recorded guitar parts from before he died and obviously Ringo and Paul's work too. Given Paul and Ringo's ages it's a beautiful send-off to some of the most famous and influential musicians to have ever lived.

You can hear Paul and or Ringo in some parts and I think the reason the vocals aren't louder is because they've lost a significant amount of their vocal range.