Are you fucking kidding me here? If you think license Gundam games comes to AC game when to combat and customization, you be so wrong. also you keep talking about visual appeal here and yet last time I checked Gundam games are also are not exactly popular in west either, are they?
I bet Armored Core 6 sold more than most Gundam games that have more "visual appeal" as you put it.
Ignore them. They are too busy defending their Dating Simulator Tabletop game. There is no denying that Divinity 3 with a Baldur's Gate coat of paint and a dumbed down DnD 5.0 ruleset is going to be very popular as unlike Armored Core the build variety isn't as complex or indepth as Armored Core or other Mecha games that put the focus on Mech parts, loadouts, and using currency to manage those parts, durability and fuel. That isn't to say BG3 doesn't have build variety just not on the level of Armored Core.
When it comes to BG3 though I still prefer the combat and interactions(elemental reactions and the use of poison) in Divinity 2.
I'd argue to say that Baldur's Gate 3 isn't as niche as some would like to think. It's a story driven party based RPG on a similar ground to Dragon Age, SW: KOTOR and of course Baldur's Gate 1 and 2. Not to mention Neverwinter Nights.
Moving on to Mecha games, it's primarily a niche franchise because it involves primarily manning Robots and in some boils down to spreadsheets to figure out a good build for a specific mission like in Armored Core or in series like Super Robot Wars it plays out more like an RTS or Fire Emblem esque game that comes down to strategic attacks or positioning.
And what gives games like BG3 an edge over Mecha games is usually mecha games do not have any form of sex appeal so are less interesting to the Western audience.
When it comes to Gundam...Gundam is boring to me. I just hate the look of the Robots as they all look very samey to me. Could never appreciate them like I appreciate AC, SRW, Xenogears, Xenoblade X, Zone of the Enders, Daemon X Machina. I just have an irrational disdain for Gundam. It's a me problem.
That said one of the best Gundam games ever made is Gundam Musou 3.
I think the issue with Mech games not being popular is that they are hard to play and mostly have tank controls that most won't take the time to get used to.