The LHC was designed to run at 14 TeV. It started at 13 TeV just a couple years ago - after two years of downtime dealing with the consequences of running at 8 TeV. It still hasn't hit 14 TeV.
The Superconducting Supercollider was designed to 20 TeV in 1976. It was cancelled in 1993. It took fifteen years to get halfway there with the LHC and another seven to get to two thirds. The FCC is going for 100TeV because, lo and behold, 13 ain't enough to do much besides the Higgs and 13 is the ragged edge of what the LHC is capable of.
The answer is more power. It's been more power for ninety years.