You're speaking THEORIES here.... in PRACTICE right now (thanks hollywood, thanks pay per view services) it is simply not.
Look at the player numbers on the recent release of total wars, that ARE CONSISTENT with the ones from paradox games:
https://steamdb.info/graph/?compare=214950,281990,394360,594570,779340,1158310,859580,203770 (click only the last year or even better the last 3 months, the data would be almost unreadable otherwise)
Warhammer, a fantasy, is EXTREMELY more popular than Rome 2.
Heck, even three kingdoms, while it was a flop, is still better shaped than Rome...
Crusader kings 2 was always a great success until CK3 took over, and now the total of the two playerbase are around the level of Stellaris (sci-fantasy)
Hoi4 and Warhammer 2 are on a league of their own, and neither is from roman time...
it's not just a matter of "good games" but in general medieval court/conquest stuff and fantasy stuff beats ancient history (by far)
huge non-sensical descrutionary wars (like ww2 or warhammer) beats everything else...
Again, fantasy is not, in fact, medieval history. Warhammer isn't even particularly modeled off of medieval history, it's early modern.
Rome is more compelling for most people than any concrete medieval society, though people are also feel a romantic connection to the ideals of chivalry (which is a very narrow period of medieval history). Rome has a far outsized influence over the public conscience than any medieval state, and the ancient period a far outsized influence on modern society than the medieval period.
I mean, Three Kingdoms was only their biggest release ever (the new Chinese market certainly plays a major factor but still), so very much NOT a flop. Rome was a flop on launch.
Rome 2 wasn't a flop either. It was just a mess and so lost people pretty hard and fast. A bit like Imperator. But the first Rome literally made CA as a company.
However, the success of Lord of the Rings and subsequent fantasy media has dwarfed the historic epic hard in the past 2 decades. It's amusing because 30 to 50 years ago fantasy was relegated to low budget B movies, while the historic epic was big premier business. There's dozen of famous HUGE budget roman epics and other ancient epics. They outnumber anything about the middle ages in hollywood. But people realized fantasy was cool, and so fantasy has overtaken the historic epic in almost its entirety really. I can't remember the last historic epic that was an actual success at the box office, or was a true everyone watches juggernaut like a GoT (or the sopranos).
Vikings have also gotten their due, in large part due to the success of marvel in the box office verse the relative failure of DC.
But, ultimately, western culture remains fascinated with the idea of rome far FAR FAAAAAAAAAAAAR more than it has any attraction to the middle ages, much to my chagrin cause there's a lot lost in translation. The popular imaginings of Rome aren't particularly accurate.
Also, like, a lot of people who look to the middle ages do so cause of the crusades, and hoo boy, you do not want to hang out with those people. The deus vult crowd sucks.