CAA regulations are all based on cost-benefit analysis and also seek to exclude sources that are basically too small to be worth regulating. In this case, the governing regulations have a complex set of criteria before the permitting requirements apply: https://www.ecfr.gov/current/title-40/chapter-I/subchapter-C...

These criteria were last amended in 2001, so the EPA is just applying a 25 year old regulation here.