Simpson is ignoring a subpoena. He doesn't want to be questioned about his relationship with Ohr, and I'm also guessing it's due to the origin of the investigation being absolute crap. Supposedly servers in Trump Tower were "communicating" with Russia via Alfa Bank, but it turns out that wasn't the case at all. The FBI started an investigation due to spam email monitoring.
https://www.washingtontimes.com/news/2018/oct/12/glenn-simpson-fusion-gps-co-founder-to-defy-subpoe/
https://m.theepochtimes.com/perkins-coie-lawyer-michael-sussmanns-coordinated-leaks-to-media-and-fbis-james-baker-likely-came-from-steele-fusion-gps_2679664.html