Because that's all most anyone makes, it seems.
Looking at the breakdown of my cellar, I have more different VA blends than any other (regardless of quantity), but if you look at the amounts... way more burley than VA in my cellar by quantity.
Low Country series, have you ever looked at it? It's like one English, one aro and four VAs, and no burley-based. There's several series like that, and various manufacturers that have 90% of their portfolio is VA-based, with burley blends being almost an afterthought.
Just like how 90% of fancy restaurants are 90% fish/seafood, or 90% of restaurant deserts are chocolate based or contain mountains of chocolate, or how 90% of the remainder of the menu at any restaurant that isn't seafood, is something covered in cheese (all of which I hate). I might be an alien...
I mean luckily there are great burley blends out there, it's not like there's no selection of them, but the number of different burley based blends pales in comparison to VAs. And don't get me wrong, I don't hate VAs either, they just aren't my top choice (they're actually my last choice... Burley > Aro > English > VA).