To be fair, that report was from 2015 and Sega was generally speaking about weak console/PC game sales in general with Alien: Isolation included, not just focusing on it. It also blamed it on a "harsh market environment" rather than the game itself.
"Despite moving 2.11 million copies, Sega calls Alien: Isolation's performance weak. It pins this shortcoming on the nebulous factor of "harsh market environment." Of Sega's six major titles released in 2014, Isolation did the best by a large margin. The other games were Football Manager 2015 (810,000 sold), Sonic Boom: Rise of Lyric and Sonic Boom: Shattered Crystal (combined 620,000 sold), Ryu ga Gotoku 0: Chikai no Basho (380,000 sold), and Persona 4: The Ultimax Ultra Suplex Hold (280,0000 sold). It's worth noting that Sega's packaged game sales saw a year-over-year increase of 1.23 million units."-
KotakuAlso, a number of games that launched with lower sales than Alien: Isolation, such as the original Dead Space, still were seen as being valuable enough to launch multiple sequels, and eventually now, after hiatus, a relaunch. By EA's account Dead Space had a weak launch with initial sales of only slightly over a million, but excitement around the new franchise was such that they continued to invest in the property for a time.
Alien: Isolation has possibly sold many times more than that through the years from release given its reputation and Nintendo Switch port. Games can sell better in the long run. Resident Evil VII initially sold 3 million copies which was below what Capcom expected, but over five years would go on to sell over 10 million, becoming the franchise's best and the company's 2nd best selling game of all time.
The perceived sales of games being "below expectations" is pretty common in the industry and often comes down to publishers having unrealistic expectations I think. Gaming commentator Jim Sterling did a good piece on this. Dead Space 2 is in the top 10 selling horror games of all time and still
sold lower than EA's expectations.
The horror gaming market has also shifted since Alien: Isolation's release. With first person horror games like Resident Evil VII and Resident Evil Village pulling in big sales. Companies often follow trends and hopefully they'll see there is still a demand for solid first person horror.