From the commercial POV, a USB scope implies "cheap" and the mfg is not able to make a lot of money on it.
Also you need a laptop, and laptops are going out of fashion.
More and more people run their entire life on a phone, which is why those of us who can read and write struggle so much with communicating with so many illiterate people
Some use a tablet, which in general doesn't do USB usefully; Apple ones are well crippled like everything from Apple, and while Android are better you still get very limited USB operation. And you get huge OS version dependencies so a USB scope may have a limited life because pretty soon the tablet which works with it exists only on ebay. I use USB endoscopes (borescopes, in aviation) which tend to be chinese made and getting them to work on a particular phone is OK for a year or two... One borescope is now "fixed" to a specific Samsung phone which lives with it in the same box, and I am sure that in a few years that $300 borescope will go in the bin because that phone will die. Tablets are also going out of fashion, with little or no performance development especially in the android sphere. Also, as I well know from aviation use, all modern tablets suffer badly from overheating... get direct sunlight on it and the internal sensor shuts it down at about +45C especially if the charger is connected, and in a DSO usage you will need constant external power. So you have to buy one of those holders which has fans built into it.
Also nobody wants to compete with the chinese; they always pick the low hanging fruit and destroy the market for everybody in the West. The smart thing is to go upmarket. DSOs exploded with cheap 1gsps ADCs which I think are down to $5 now and everybody and their dog in china is churning out DSOs with those ADCs, and if you want to make a DSO with say 5-10gsps then you are spending real money on the ADCs and on other hardware, and making it USB-only just tells potential customers that you are "cheap".
Also a dedicated self contained box is nice to use. I have a Lecroy 3034 here and while the OS is sh*it and slow as a pig, the whole package, with nice knobs, is quite good. I can even access it over RDP (remote desktop) or the LC application can run over a VPN (poorly but usably). That scope was about 5k and it will work for many years. Well... it blew up after 1 year and got fixed under warranty but has been ok since; I leave it running 24/7