I would guess: age, code availability, inertia, and lack of benefit.
PBKDF2 is old, and had time to get adopted. Code is widely available for it, even included in many frameworks. Once you implemented something like this, unless it's really broken it's a lot of hassle to change it.
I fairly recently looked into implementing password hashing, and my requirement was that somebody already did the job for me, because I'm not a pro cryptographer and don't trust myself to do it right. So since Qt supports PBKDF2, PBKDF2 it is.
And Argon specifically is RAM hungry, which limits applications on things like low end virtual machines and mobile devices.
PBKDF2 is old, and had time to get adopted. Code is widely available for it, even included in many frameworks. Once you implemented something like this, unless it's really broken it's a lot of hassle to change it.
I fairly recently looked into implementing password hashing, and my requirement was that somebody already did the job for me, because I'm not a pro cryptographer and don't trust myself to do it right. So since Qt supports PBKDF2, PBKDF2 it is.
And Argon specifically is RAM hungry, which limits applications on things like low end virtual machines and mobile devices.