- Ability to do some scripting, or integrate it into a product/project.

- Webserver/standalone operation.

- Integration with other standard comm buses.

- Option for Better A/D (typical o'scope is 8 bits).

- More/better output options. You could potentially use it to simulate some hardware (in the loop).

- Ability to add COTS hardware.

- Open design insures owner against planned obsolescence.

This is kind of like asking why there's more than one model of car on the road. Other than wanting to carry more people or go faster, what differences are there? Any automaker could just produce three vehicles: a sports coupe, a panel truck, and a bus. Maybe a motorcycle (again, just one model).

Depth of the sample buffer. Tradeoffs between sample speed and sample resolution. Triggering. Math (anything from sums/differences to FFTs). Bandwidth, including tricks like equivalent-time sampling. Various kinds of display. Various kinds of human interface. Various kinds of machine interface. Extra features like waveform generators.

Honestly you can figure this out yourself: go to some scope makers' websites (Tektronix, Agilent, B&K, Fluke, Instek, Teledyne Lecroy, etc etc) and see how their marketing department distinguishes their product lines.