To save cost. That is the only reason now.
You can buy fully isolated scopes now - especially with the advent of fully digital scopes, they can literally have an isolated ADC unit for each channel which then transfers the waveform data digitally over a digital isolator with zero loss in quality/linearity.
Fully isolated scopes are far superior in all ways (except cost.) You don't have to worry about ground path loops between the shields of neighboring probes, or between earth ground and your circuit under test. You can simultaneously measure the voltage drop across multiple points when none of them are ground referenced on either side.
But why is non-isolated so prevalent if isolated is so much better?
Back in the olden golden days, scopes started out being single channel, and big, and using primitive components. It was quite a feat of engineering just to get the scope to respond linearly from 0 to 100Mhz.
Isolating an AC signal is easy - just use a transformer. Linearly isolating a DC signal is much more difficult, although possible using opto-isolators and feedback loop, or a voltage to frequency converter and an isolation transformer -- but that doesn't work for 100Mhz because you'd need Ghz frequencies.
Remember, the oscilloscope was invented about 120 years ago. What is easy - or at least plausible now - was not back then. Isolating linearly from DC to even 1Mhz using tubes and such was basically out of the question - and since most scopes were single channel, or maybe 2, technicians had to just deal with the fact that both probe ground leads were connected to each other as well as to ground, and the scope was still extremely useful with this constraint.
That of course set quite a tradition of "We've been doing this for a hundred years, we'll keep doing it that way."
But nowdays non-isolated scope design is just stupid. Scopes are often 4 channels, and we often need to measure voltage differences in circuits which are not ground referenced - and the technology is totally here now make fully isolated multi-channel scopes.
Modern scopes just convert the incoming voltage to digital anyway. At that point, why not pass it through isolators and have an isolated scope?
But people are lazy. Rather than put the effort in to update their designs, they just apply minor adjustments from last year's model and call it good.
At some point a leading manufacturer will just go all-isolated (for the same price) then the game will be over, all the manufacturers will have to go full isolated or go away.