What a silly reason, it's always worthy.

What Yogi said.

Becasue not all anime is 1080p worthy.

Probably because of the time required. Depends on what kind of computer you have but the time required to encode a 1080p video is a lot longer. Unless you have an encoding server or something.

And the size of the file is large as well.

Fansubbing is generally just done as a side hobby by a lot of these guys so they may not necessarily want to invest much more time into it. Not to mention the fact that fansubbing is kinda dying out.

does anyone know why most of the BD releases are 720p and not 1080p? I thought that BD native resolution was only 1080p