[AstralStorm]

It is quite a bit easier to hold back a liquid than a gas. Different minimum pore size and you get hydrophobic chemistry and surface tension on your side.

Especially at such big pipe diameters as required.

You can use cheap materials like modified concrete to patch a hole in an underwater tunnel, not so in a vacuum tube - that needs something much better and more expensive.

[Robotbeat]

Not true. You can use concrete to hold back a vacuum and cheap materials like a thin polymer coating, too. Remember, Hyperloop does not require an ultra hard vacuum.