Why modern oscilloscopes inputs are not fully differential (i.e. floating)?
Differential probes usually used to monitor very fast signals with relatively small amplitude. In such case diff amp must be placed as close to the source as possible. Proximity is a key feature.  

Also, I'd like to point out that single-ended isolated input usually is not equal to differential input. It's easy to make 1500 V isolation of the whole single-ended circuit, but it's utterly not so easy to create fast diff amp with such allowed common-mode voltage.