Alien and Prometheus are intended to take place in "The Future". I'm sure there are years assigned or whatever, but they are pretty unimportant other than of ordering events and placing them in The Future. Alien wasn't intended to portray an alternate future in which we never progressed beyond CRTs or regressed to CRTs or whatever; it's just that that's what was available (within budget, feasible, etc) to the filmmakers. If you gave the Alien team easy access to flatscreens or CGI-holograms back then, I bet they would have used those instead. Maybe they would have had them flicker or whatever, to demonstrate that the Nostromo is a hulking rustbucket, but they would have picked whatever they could that would indicate to viewers "This is clearly taking place in The Future".

The filmmakers of Prometheus have the same choice. Using CRTs, while consistent with the look of Alien, would have

Implied that these films take place in some sort of future/alternate-history in which we never progress beyond or regress to CRTs
Probably confused viewers. Not that they would have doubted it was the future, because spaceships, but it would have been weird.

The filmmakers either assume that the viewers can get over this discrepancy, being aware that Alien was made in the 70s, or are super-committed to not-being-George-Lucas (by which I mean making Special Editions), or both.