Question

When did it become common for photographs to be collected in major fine arts museums??

Answer 1

It became common for photographs to be collected in major fine arts museums in the 1980s.
This was the time when photography really started to be considered as "art" and thus was given its rightful place within museums so that people could admire it for what it was. Photography doesn't only refer to family pictures and such, but can show real pieces of art, which was why it was ultimately included in the museums during the eighties.