Murphy and Walter were ineligible to be traded by DD as they were drafted the year he was fired. Crawford barely reached AA before DD was gone and was a middling prospect at the time. Houck was an afterthought going into 2020 and had already be pushed into a bullpen role. In 2019, Bello had a 5.43 ERA in Low A and was ranked outside the team's top 20 for most of the year.
It's not like DD was looking at these guys and thinking "boy, these are the pitchers I need to hold onto." The most interesting pitchers when DD was fired were Mata, Groome and Song.