Why do 10gbe network adapters have twice the PCIe lines required to pass 10 Gbit?

Intel’s x520 and 540 or Mellanix MNPA19 were 8-line PCIe 2.0, which is 500 MB/s per line i.e. 4 GB/s.

x550 is 4-line PCIe 3.0 or 1 GB/s per line i.e. 4 GB/s.

Both are 2x wider (or 4x wider?) than the number of lines necessary to pass 10 Gbit.

Why is this necessary?