Why was StDr56 discovered only now? StDr56 is a newly discovered planetary nebula (maybe). link1 link2
It was found by amateur astronomers Marcel Drechsler and Xavier Strottner.
According to the above articles, it's pretty big:

With an extension of 44 x 36 arc minutes, StDr56 is not only the
largest probable PN in the region, but in the night sky it covers more
than half of the area of the famous Triangulum Galaxy M33, which is
located only a few degrees from Strottner-Drechsler 56.

Or

It’s about the same size as the full Moon on the sky.

So, if it's so big, why was it only discovered now? Wouldn't it make sense if all large objects were already discovered a long time ago?
Large objects can be very faint if they are far enough away. So large objects wouldn't necessarily be discovered a long time ago.  The object you mentioned is very faint and required a long view time to acquire enough photons to &quot;see&quot; it. As stated in a comment by @Pierre Paquette, the object was viewed for over 59 hours.