The general rule is that the BX2380 is the standard machine. It has enough power to do everything - it's the model they base the TLB BX23s on (the backhoe version). It has the same hydraulic system as the BX2680.
The BX1880 is the economy version. Not much cheaper, but it's a price leader. It gives up a bit of wheel size and a bit of power, and it is limited sometimes in what it can run - smaller mower, less PTO power.
The BX2680 is the "more power" version. The only place it can really use that power is out the PTO, and maybe dragging things uphill, or running the big mower in heavy grass. But most of the time the BX2380 will do what the BX2680 does. It's not a lot more money for the 26hp, but there aren't a lot of uses for that HP in the BX size machine.
That's ultimately why the BX2380 is the biggest selling machine.
The BX1880 is the economy version. Not much cheaper, but it's a price leader. It gives up a bit of wheel size and a bit of power, and it is limited sometimes in what it can run - smaller mower, less PTO power.
The BX2680 is the "more power" version. The only place it can really use that power is out the PTO, and maybe dragging things uphill, or running the big mower in heavy grass. But most of the time the BX2380 will do what the BX2680 does. It's not a lot more money for the 26hp, but there aren't a lot of uses for that HP in the BX size machine.
That's ultimately why the BX2380 is the biggest selling machine.