NEXRAD—why is it 2D? If you paid attention to NEXRAD training, you know that the colors of pixels show the maximum activity at any altitude in that air column. (It’s a “composite” view.)

NEXRAD itself samples in three dimensions, so that integration into the composite is being done on the ground before being sent back out on SiriusXM or ADS-B.

Given ADS-B’s bandwidth constraints, it makes sense that you only get maximums—it’s the thing that most pilots will care about. The bandwidth constraints are the same reason pixel granularity falls off with distance—the stations can’t send nationwide data in the bandwidth allotted.

But could SiriusXM give you the raw 3D data in addition to the composite view? They have the bandwidth where you could dial in a given altitude and get a 2D slice rather than just maximums. And you can get other data like wind on an altitude basis, so the capability seems to be there.

I don’t know if it’s a case of cost (considering how SiriusXM charges for subscriptions, I imagine they’d be happy to offer this to pilots as an additional tier), a worry that some pilots could take on even more risk around storms by trying to do 3D rather than just 2D daisy-clipping, or something else (like a technical issue with NEXRAD—as far as I can tell, none of the weather sites offering NEXRAD give the ability to dial in altitudes).

Given how many accident studies (even into incidents where the PIC’s were instrument-rated private pilots) have shown that pilots who flew into unexpected IMC or icing could have survived had they made a different decision about what altitude to seek for safety, it feels like it could be an improvement in situational awareness and safety, and usually we seem to accept those technologies in aviation, even if they may also present greater risk of recklessness.
There is not much gained by the third dimension for your use case. In GA you generally want to avoid any active weather. Why make it more complicated? 


can't go over it (performance limited)
can't go under it (microbursts can kill you)


=> only way is to go around it and therefore no need for that 3D information.