I want you to take a look at what are the top threats in this gen. Notice how celebi is eaten alive by almost all of them. Genesect, lucario, charizard, talonflame, aegislash, excadrill (swords dance), heatran, garchomp (any set except choice scarf), gengar, greninja, landorus (both formes), pinsir, venusaur, scizor, kyurem-b and there are stuff that although cant do much to it, celebi cant really prevent them from doing their job like ferrothorn and both deoxys formes. Dont even get me started on the other great threats like mandibuzz, volcarona and bisharp. Like seriously, unless the metagame becomes completely centered around rain again, i dont see celebi getting any relevance anytime soon. The meta is just harsh for it, and honestly venusaur is just so much better in the role of a bulky grass. It pretty much walls everything celebi does and even more. Sure it has nasty plot+baton pass, but when are you going to find time to setup AND pass considering that almost eveything in the tier forces celebi out? Honestly its just too niche even for B rank, specially considering that this gen B rank has stuff like clefable, the aforementioneds bisharp and mandibuzz, mamoswine and other excellent mons which celebi doesnt really compare to in terms of usefulness.