From the 1970s through the 1990s, almost all porn was recorded on VHS tape. Even a well preserved master that is converted to digital is restricted in being "poor quality" no matter how well it's done. This is due to the poor quality of VHS recording in general.
Even digitally enhanced remastered copies of VHS recordings look "old" because the original copy was low quality from the start.
Not to mention that magnetic tape (which is what VHS is) degrades over time, even in the best of environments. Porn studios weren't really known for keeping their masters in the best possible location inside their offices.