Why so few osilloscope with dedicated isolated channels?

Why are there so few bench osilloscopes with dedicated isolated channels? Is it a design issue, a cost issue, a business issue or is it just better with differential probes?

It is a cost (price) and demand issue. Most users don't need isolated inputs. Most users have ground-referenced signals so a scope with non-isolated inputs is OK.

Cost is an issue as isolated inputs require more complex circuits compared to the standard solution. This makes the oscilloscope more expensive.

Also note that isolated inputs and differential probes are different things! You can use an isolated input as if it is a differential (floating) input but it's not really the same as the connection on the oscilloscope is a BNC connector which is a single ended input. The two electrical connections on a BNC plug aren't precisely the same, one is also the shield, the other (inner connection) is shielded. For DC and low frequency that usually doesn't matter much but for higher frequencies ore low noise measurements it can matter. Then a true differential probe is the better option.

Source: I work for a scope company with scopes

Essentially, the answer to "can it be done" is almost always "yes" unless it's a physical impossibility. However, the number of people that actually need this and make it a purchase requirement isn't really that high. Often there are workarounds that folks find.

There's also some discussion in this thread about differential probes vs. channel isolation. If you want to learn more about differential probes, there are a couple killer videos on our YouTube channel about them:

Single ended vs. Differential probes: https://www.youtube.com/watch?v=a28lVMsWFEY

Differential Probe Guide: https://www.youtube.com/watch?v=OZDijMDHmtI

One way to think about it is that an isolated measurement is a differential measurement, but a differential measurement isn't necessarily an isolate measurement. You can actually use a differential probe as a single-ended probe, which is covered in that first video.