The reason is that they are smaller and if you want to make an example to scare an industry into compliance it's better to go after big companies first. If Facebook gets dragged over the coals the smaller ones are next unless they adapt. This myth that only non-EU companies get pulled into court is nothing but propaganda, mostly spread by the poor, poor, US-based violators of privacy.

https://www.enforcementtracker.com/ has a list of enforcement actions, and low and behold, most of them are against EU companies.

(The country filter is for the fining entity, not the fined entity, just in case anyone thinks this doesn't include US companies at all)