I am especially interested in fan theory on why shield doesn't work in subspace, and why subspace looks swirly.
In-universe head-canon:
1) Subspace contains an energy field that creates a destructive interference with the Shield.
2) Because its not the ships that are moving but the transit corridor.
Meta-level:
1) Because the Lucifer needed to be destroyed somehow and they were not going to go down the "oh, it just takes lots of bombs"-route because that would not be approriate to the theme of "destruction of the old bringing forth a better new". If it were just a question of firepower the entire inclusion of the Ancients in the plot of FS1 would have been redundant.
2) Because it looks unfamiliar and is disorienting; yet also its motion translates to the idea that the Lucifer is moving when it cannot for gameplay reasons (otherwise the Ursas would take forever to catch up).