reasons why it underperformed

36m opening - way below Prometheus (51m), Fury Road (45m) and even below 2004s AvP (38m- although that was PG13 lol). And now looks like it won't even reach AvPs 80m total domestic - http://www.boxofficemojo.com/franchises/chart/?id=alien.htm
on the plus side it cost a lot less than Prometheus (97m to 130m) is more favourably reviewed. Plus is #2 on the Alien worldwide scale (the others obviously not adjusted for inflation however) and still has China to come..

Anyway reasons are thus:
-not as much as an event this time (Prometheus had the anticipation of RS's return to SF for first time in 30 years for his 3rd SF film after the iconic Alien and BR. (unlike now where he's since done Prom and Martian was only 2 years before) And his return to the Alien franchise since 1979. plus at same time was anticipated as an intriguing mysterious original SF thriller that was rated R when PG13 was the norm and had that incredible trailer.)

-no A list film star this time (Prometheus had Theron. Fassbender not really a draw on his own as lead yet)

-audiences burned by Prometheus (which had seriously mixed reviews so the turn out was much less for the sequel. Be like if they made a sequel to Burtons Apes or Superman Returns. Or see Attack of the Clones box office drop after Phantom Menace or AvP2. Tomb Raider 2,TNMT2)

-The 2nd middle chapter in SF trilogies tend to make least BO (see Empire, Temple of Doom, Clones)

-audience burned by 'Alien?' (Ridley has said they made it 'Alien'/xenos due to fans sounding off about it on SM etc so its ironic to think that making the sequel into a full on 'Alien' film under the assumption that would equal more box office/greater audience satisfaction may have backfired somewhat. Even though putting 'Alien' in the title as opposed to 'Prometheus: ...' or 'Paradise' or even 'Covenant', and promoting the xenomorph in posters/trailers as a reaction to the Prometheus negativity would seem a plus, the last 4 Alien films with 'Alien' in the title disappointed just about everyone. also 'Alien' and the xeno denoted it was fully returning to its gory horror roots so in the eyes of the non geek audience with just a vague knowledge of the franchise who view Alien as just slasher films Freddy/Jason/Halloween in space it might've turned them off especially families/teens. and downplaying it being a Prometheus sequel might have inadvertently created confusion/turned those off who actually liked the more SciFicentric Prometheus)

-5 years since Prometheus (maybe the norm for Alien films but people are now used to only a couple of years if that between films)

-no 3D (unlike Prom - although 3D was more of a 'thing' back in 2012, not so much now)

-mixed reviews (again. like Prometheus. not an instant classic like Alien/s)

-overseas release a week before US (allowing everyone to know all spoilers on the net, the mixed reviews etc and think 'meh will catch it on dvd')

-fierce May competition, opening between established family blockbusters Guardians2 and Pirates5, plus King Arthur and Baywatch, and then theres WonderWoman -unlike August where AC was originally scheduled - really just The Dark Tower as competition. (Prometheus didnt have much in the way of competition when it was released in June 2012). maybe October wouldve been better for Halloween (but then would be too close to Blade Runner – ideally it shouldve been BR2 in May and AC in Oct)

-Star Wars (since Prometheus in 2012 Star Wars has come back in the biggest way possible and kind of blown all other retro space franchises away making them less of an event - also see last years Star Trek Beyond)

-Blomkamps nostalgia fuelled Weaver/Biehn starring Alien 5 (the excitement for fans seeing the artwork, Weaver/Biehn/Cameron even Scott saying its happening script is great etc, perhaps dampened enthusiasm for 'Prometheus 2' - esp when it was revealed it put the block on 'Aliens 2.' 'Aliens' really turned the franchise into a SF action series not so much slow burn horror of the original and it became more associated with that in comics, games etc and won a huge fanbase due to it. yet strangely the films have never really attempted another non stop action 'Aliens' with Marines/Pulse rifles vs hoards of xenos, as the nefarious Company pulls the strings. Theres been half hearted attempts - elements of AR and the AvPs but nothing like a proper effort..and when Ridley returned he's building on the mystery of the Space Jockey and the alien origins..not so much military action stuff/xeno army. So when Blomkamp showed he was planning a return to all that fans were understandably pumped..only for Covenant to take it away..- in fact it would appear the positive fan reaction to Alien 5 helped prompt Scott/Fox to really up the xeno action in Covenant as stated above)