Blu-ray is also compressed. At issue is the amount of compression. A 4K movie compressed to fit into a 15GB download will be very highly compressed, but the perceived increase in resolution and sharpness could make it look "better" in some situations than a less compressed 1080P BD.
They are feverishly working on more efficient codecs that can compress more efficiently but it's at least a couple of years before the 35-50GB BD quality can be matched by a much more highly compressed file. It will also require new hardware that can play files encoded with these new compression algorithms.
The final footnote to all of this is that 95% or more of consumers could care a whit how compressed a movie is, because they don't notice banding, macroblocking, poor black levels, low color depth, etc.... usually because they have very poorly setup equipment that they sit very far away from. 10' away from a 42" screen you aren't going to notice jack about quality problems and would have a hard time telling BD from DVD anyway. That's the #1 target demographic for streaming I imagine, behind those who consume content on tiny phone, tablet and laptop screens.