Their core game design feels like a straightforward iteration on mid-to-late last gen ideals when we've already passed the cross-gen era and are halfway through this generation.
There are some genres that can get away with this easier because they're very much in vogue, but I don't feel it applied to what either of these products did.
Tomb Raider is a somewhat open-ish singleplayer cinematic third person shooter, which is a product you see almost no one making anymore. They also didn't deliver the level of spectacle and production required to try and make the product really stand out despite no longer being in a popular genre, and there wasn't a highly compelling multiplayer mode that fed into current trends to pad over that issue. Frankly, sometimes genres just stop working over time at the same price level because consumer tastes change or they just keep on expecting more and feel that the genre in question no longer delivers it. They can also just reach the point where they feel they've had their fill and need a really inventive reimagining to care again.
Deus Ex just looks exactly like the 2011 game with improved graphics and some tune ups whenever you look at one of the demos. We can try comparing this to the RPGs that are really selling these days. If you stack it up next to The Witcher 3, it looks ancient in ways that are not just the graphics. If you put it next to Fallout 4, you're missing out on all the little things Bethesda did to sell people on that ranging from settlement building to extensive crafting and customization systems to lots of focus on refined and improved moment to moment combat to the expectation of world scope they've built up with their audience over their past six games. If you put it next to any of the multiplayer games like Destiny or The Division, it's a singleplayer title. If you say "Actually, Deus Ex is a stealth game.", the product looks way less impressive in gameplay and scope than Metal Gear Solid V. I don't think anyone expected the game to play nearly as well as Dark Souls 3 or offer the level of imaginative world design that the Souls series offers, so that comparison doesn't work out either. It becomes a product that's very hard to build excitement for. While it still sold pretty well, EA similarly struggled with this with Dragon Age Inquisition, where it was very difficult to build excitement while offering a product that looked like something people had already experienced. However, it was also one of the first major RPGs out this generation, which helped a lot. You will notice the way they sell Mass Effect is more about showing you driving across gigantic planets, having very snazzy visual effects, and trying to impress people with high concept fantasy fulfillment around exploring a brand new galaxy. Horizon is similarly something that's trying to sell an exciting idea, a large open world, high end visuals, and what looks like sharp action gameplay.
I feel Final Fantasy XV's problem comes in the same form, where it essentially looks like a mixed bag in demos. Just giving an example palette of reactions, we could easily have someone sitting there going "Wow that monster looks awesome. These characters look kind of dorky. This combat seems wonky. Wow this spell effect is awesome. These quests look really boring. This town is great. This dialog is cringeworthy." and then proceed to look at their $60-$240 for this Fall and end up picking up a bunch of other twenty different games releasing this Fall instead, or maybe just spending it on some DLC or microtransactions for the games they already like. I really feel that's what happened to Tomb Raider and Deus Ex, and why they did so much worse than their predecessors. People just felt they had more exciting options available for one reason or another - or a large combination of them - and decided they'd just buy those games cheap if they still felt like it later.
It's certainly possible I could be wrong. Maybe everyone who looks at the game has been completely blown away and is pre-ordering through the roof right now, but based on what I've seen over the years and the trends I've observed, this strikes me as a game in the "This is an uphill battle." pile.
Edit:
Since I didn't write as much about Tomb Raider, let me use Rainbow Six as an example. That was going to be a mission based, singleplayer heavy, kind of open but also with setpieces shooter and was instead rebooted as a multiplayer only game, and is one of Ubisoft's shining jewels this generation that they brag about at every possible opportunity. Ubisoft saw the writing on the wall for that one and acted appropriately. Splinter Cell Blacklist was a really wonderful game, but not one cared, because that type of game was on its way out as a $60 product that appealed to enough people to appease modern AAA budgets. Instead of trying to brush the game up and release another one, Ubisoft sent it back to the drawing board, the same way that the linear co-op third person shooter Ghost Recon Future Soldier - which actually did pretty well for what it was - was followed up by a massive open world Ubisoft game that still had the co-op angle. The game also has much better production values, extensive character customization, what looks to be way more mission variety and gameplay approach options, and they've teased that there will be a large scale dedicated and supported competitive multiplayer suite they'll be unveiling later. I'm sure it cost at least three to four times as much as the last game, but that's what the market wants.
I know this will sound really harsh, but for Tomb Raider, I think they needed to reinvent the entire series again, because it took too long for them to get the first reboot out and they already lost the opportunity to make an iteratively improved sequel instead of a striking reinvention toward modern trends.