> Why can’t they just use a Cygnus to do it?

To small. You'd need something like fully loaded ESA ATV to deliver enough delta-v. Single biggest push ISS ever got was from https://en.wikipedia.org/wiki/Johannes_Kepler_ATV and with 4.5t of fuel (total theoretical payload capacity was 7.5t, but part of that was fuel for ISS, water and dry cargo allocation) it delivered less than 30m/s delta-v, raising orbit form 350 to 400km. Here you need about 50% more. So theoretically fully loaded with fuel it could make it. But that was the biggest resupply craft flying to the ISS.