This is my feel on the matter as well. Giant robots have a certain mass appeal that fighter jets (or spaceships) lack. The #1 physical game title on indie site itch.io is Lancer, a mecha RPG, and it's been there for literally years. It's the visceral power fantasy appeal: a 'Mech is you, but bigger and better. Even the alien-looking machines like the King Crab still have recognizably anthropomorphic characteristics. You can get up-close and really lay into your enemies, and if guns fail, there's always your big steel fists. But a fighter is a pointy tube with wings and WarShips are even more impersonal. You need to be into military hardware and tactics already.
I fully believe that AeroTech should see a new edition of some sort, with the next iteration of the core rulebooks moving fighters and spaceship combat into their own books. But I wouldn't hold my breath for them to be massive breakout hits.
I don't think that holds up if you look past just physical game titles. While there's certainly a big market for 'Mech games like Battletech and Armored Core, there's huge markets for both terrestrial and space combat: Star Wars alone has a ton of titles that focus on dogfights or space combat in general, the Ace Combat franchise is huge, there's no shortage of strategy games where the Space War is the primary (or even only) theatre you play in. I don't think most of these can be said to have niche appeals, at least not compared to 'Mech games, but of course they're not going to be sampled when only considering physical game titles.
Of course, limiting it just to physical hex-based games, you're absolutely right: In that particular area, fighter & fleet hex games are quite niche.
My guess as to why BT Aerospace is relatively unpopular is:
1. Battletech, the Board Game of Armored Combat is, first and foremost, a Battlemech centric setting. Consequentially the 'Mech part of the rule set is very polished, and other areas tend to be less polished the further from 'Mechs you get.* No one really gets attracted to the setting
just for its space battles; if people get interested in that part it's usually because they were also interested in the 'Mechs which drew them in first. So there's a minimum of two different areas you have to be interested in before you can even begin to consider playing a Battletech Aerospace game: Aerospace Fighters / Warships AND Battlemechs (arguably 3 things, if we include the Battletech setting itself). That by itself is going to be a higher bar than settings or game that are fighter-centric or warship-centric, in which case there's only 1-2 bars of entry: Being interested in dogfights, and possibly the setting itself.
*ASF and Warships in general suffer from this to some extent. A simple example is how BV doesn't reflect weapon utility in space nearly as well as it does on the ground, and the primary culprit is simplified range brackets. For instance: Clan Heavy MGs have a BV of 6, while Clan AP Gauss Rifles have a BV of 21. Both have 3 points of damage, and the HMG deals 3D6 vs infantry while the AP Gauss does only 2D6. The BV difference is justified on the ground because the AP Gauss has nearly 5x the effective range of the HMG, but in space both occupy the Short range bracket and perform identically (except the HMG strafes slightly better). That is, an ASF AP Gauss costs 3.5x more BV than a ASF HMG for identical performance. This is a clear artifact of the Battlemech-First setting.
2. Battletech, the Setting, is, first and foremost, a Battlemech-centric setting. Compared to the ubiquitous Battlemech ground battles, there's not a ton of in-universe places where Warships actually play a role beyond plot-necessary orbital bombardment, and even fewer places where the Warship engagements are meaningful. In fact, in some eras they're all but written out: The IS Warships get obliterated during the Succession Wars and the Houses don't/can't rebuild them, so the Clan Invasion era starts with the Clan warship fleet having totally free reign besides the occasional IS pocket warship or planet-based fighters. In the Dark Age they're basically gone again, with IS powers having low single-digits of warships at best and the Snow Ravens possessing the lion's share of humanity's warship power with maybe a dozen, mostly in mothball?
This is a major hurdle for me. I really can't run a canonical Dark Aged aerospace game. I can't run something like the Draconis Combine Invasion of the Federated Suns because DCS
Draconis Wind is more than half of their entire Warship strength. It'd be at best a Pocket Warship slugfest, and the players I have that are
conceptually interested in fleet battles and enjoy them in other settings or mediums are not interested in such a campaign even if the Aerospace portion of the game was mechanically flawless because they're interested in
ships, not
boats.
3. The physical game style generally doesn't lend itself very well to simulating atmospheric or space fighter dogfights or fleet battles. There are some, but they're not common and generally not as well known, or popular. Hence, the group that are interested in such things go to the mediums that do have them and do them well, like the video game format. This factor probably heavily influences physical, hex-based, fighter-based or fleet-based games being very niche.
