A couple of points I would like to make:
* Structural requirements
The hyperloop has much less structural requirements than a traditional train. The reason is that its cars are separated, so it is 'less dense'. If a train goes over an overpass, the overpass has to be engineered to carry the weight of the rail and the train, whereas the hyperloop only has to handle the weight of the tube (30m between pylons at about 8 tons/m is 240 tons plus the negligible weight of the capsule).
The upshot of this is that the pylons can be much less invasive on the real-estate requirements. This is important because now you have more choices with regards to routes, which leads to less curvature and higher speeds. Looking at it from the reverse angle, High Speed Rail design suffered because the massive real-estate requirements imposed such a burden that route choices were compromised, and thus the projected travel time was lengthened.
* Tube manufacture.
It occurs to me that there already exists expertise in manufacturing elevated, highly reliable large diameter steel tubes hundreds of miles long. Oil pipelines. Moreover, these companies perhaps would be interested in diversifying their business away from oil.
* Development
High Speed Rail has issues, but they are political and financial, not technological. We're really just buying the technology from other countries. Much of the cost and the incredibly lengthy construction time for HSR is coming from building overpasses and the foundations for the rail, so far as I can tell.
But hyperloop is something that requires some development. This is a good thing because investors can sell that technology and get some return. It's difficult to attract private funding for HSR, but I can definitely see someone stepping forward to fund the development of the technology.
Pretty much once someone builds the hyperloop demonstration system, you'll know if you have a winner on your hands, but with HSR, you don't know if you'll be successful until the whole thing is built (and with a 2:30 travel time LA->SF, it's not going to be a slam dunk against air).
There are plenty opposed to HSR, but I see very few alternative solutions being proposed. This might be something those opposed to HSR jump onto, and it might get a lot of support quickly.