Marketing purposes. I do have to say the people behind the marketing especially the ones behind picking the names were quite brilliant. They actually knew what they were doing.
You see the name "The Last" suggests it's the last movie of it, the end. So of course people will fall for it. The ending to something most of the time brings in more people because it brings back the ones that dropped it before and now they want to see how it ends.
Similar thing with "Boruto". Kishi said that if Naruto has a kid, the kid's name will be "Shinachiku" but Shinachiku doesn't really sound that good in term of marketing now does it?
Think about it "Boruto" - "Naruto". The names are close; so people will be more likely to go watch a "Boruto" movie instead of a "Shinachiku" movie.
Seriously? I'd be talking about how flawed that logic is right now but it's meaningless at this point...
But in all honesty, this is not something intended to attract loyal fans that have been following Naruto for a long time and know the story. If anything, it's an insult to them, kinda like getting called stupid in front of your face. This is just something to attract certain fans that wanted to see specific things happen (like NaruHina) while altering the series facts, character personalities... EVERYTHING, in order to justify what cannot be justified in the first place.
Oh and Shinachiku? Love that name and I would totally watch the movie, as long as it stayed true to everything that was shown in the past years.