I really think it’s just the right frame of mind. I can easily forget why I opened some app on my phone, but yet I thoroughly enjoyed Alien, as well as 12 angry men. But these are not movies you just pop in and half-ass over, they require real concentration — people just often unwilling to commit their attention to one thing, not incapable.
I grew up watching movies with slower paces (born 1980), but some of those I also have issues watching unless I'm in the right frame of mind. I think in part it's just we've gotten used to information overload, and thus expect the faster pacing.
That said... What I really like about the original is that it has that classic look and feel of 70's sci-fi. The white paneling and bright lights on the space ships, the grainy film, red and orange stripes on things, etc.