Well as you probably know (but maybe not), Sherlock just won emmys for best writing, best actor (ben c), and best supporting actor (martin). So basically people are excited because Sherlock is getting massive critical acclaim for both its writing and acting even though a lot of casual viewers found series 3 to be poorly planned and poorly written in terms of character development. HOWEVER (and this is just my opinion but I believe a lot of others hold the same one), to those that believe Johnlock will eventually become canon, series 3 is just the next logical step in that story so seeing it get so much critical acclaim somewhat cements the idea that the writers are planning to make johnlock canon. Not to mention that now that Sherlock has won awards in pretty all its categories, canonical johnlock would be an even bigger step regarding queer representation in popular media.
(I just typed this up in like 2 minutes because I need to take a quiz so if anyone along the lines of joolabee, thealogie, or wsswatson would like to make more sense of my babbling please do!)