I suspect it was a combo of a few things: a lack of space in the 58s innards to fit new equipment and the fact that they suffered chronic wheelslip issues, especially in comparison with the other heavy freight Type 5s (56 and 60), which both were a lot more sure-footed (why the SEPEX wheelslip control system 58050 had fitted wasn't perpetuated across the entire class I'm not sure). The overall numbers - 50 58s compared to 135 56s* and 100 60s - likely factored in with this.
Even with all their problems/drawbacks in mind, I'm glad a few examples have escaped the cutter's torch and have been secured for preservation. Much like the less than successful members of the pilot scheme in that sense (I still wish a 29 had been saved though, but that's neither here or there...)
*: I'm aware that not all of them made it to privatisation, but the general point about their overall numbers still stands.