Some times Poskim give us a glimpse of how they arrive at decisions
Here Kitzur SA illustrates 3 methods:
1. "Haskamat Rov Happosqim"
Sh'mittat haK'saffim is noheig nowadays [hutz la'aretz]. See Hinuch on "acharei rabbim l'hattot"
2. Some poskim were "tarchu l'lameid z'chut." While mainstream Halachah follows #1, it's still good to avoid condemning meikilim by limud z'chut
3. "Uvifrat sheyuchal l'takkein."
No one should use a kula when the alternative is a simple, cheap option/fix to write a prozbul!