There are two types of people who bought Bannerlord. Those who played Warband and those that didn't. If they had never played Warband then they bought the game because of the marketing and hype that was out at the time of the game. If they played Warband, they bought Bannerlord because of the legacy of amazing total overhaul mods that were and are prevalent in that game.
Native Warband, while fun, was an entirely forgettable game. It was worth 40-50 hours of play and that was it. The reason I have over 2000 hours played in Warband was because I saw a mod on Youtube and my jaw dropped. Form the second I installed the Mod I was hooked and there has been no game like it ever. Never have I been able to buy one game but play a couple dozen different ones. I played Game of Thrones (2 versions of it), Lord of the Rings (2 versions of it), Warhammer, 3-5 completely new fantasy IPs such as Prophecy of Pendor and Persino, a few mods that just enhanced the Native experience, at least 10 historical variations ranging from a 1500-1600 AD Renaissance version to Feudal Japan to the Crusades to the Napoleonic period to Vikings to one mod even set in the areas of South East Asia like Vietnam, Cambodia and Thailand. Hell there are even sci-fi mods like Star Wars available for Warband. That is why I have 2000+ hours into Warband and why I bought Bannerlord. Bannerlord Native is the same as Warband. 40-50 hours then done. Probably still a buy if there were no mods but I am expecting another 10 years and thousands of hours out of Bannerlord mods.
I also think this is why anyone who played Warband bought Bannerlord despite no existing mods at the time EA released, because we know (hope) that eventually there will be.
As for why so many people on Steam never used mods, well if you haven't played Warband, you might not even know that the game has mods. I mean EVERY game that can be modded typically has a Steam Workshop where users and can upload and download directly from Steam. However for some odd reason, Bannerlord doesn't have a Steam Workshop. Maybe they are waiting for a full release but it honestly wouldn't surprise me that anyone new to the franchise is pretty clueless about how important mods are to the success of this game. The problem is now it might be too late for most of those players because they came, the saw, the played their 40-50 hours, got bored and now this game isn't even on their radar any longer. Many will probably never figure out that the best, most enjoyable gameplay this game has to offer is through mods.