Most top players will partly say because he's over-reliant on wonderwing, but that's not really the case with good players that know how to reserve its uses or even confirm into it from breegull blaster.
Idk that I go so far to say he's low tier, but what holds him back is terrible airspeed (iirc, I think it's 6th worst in the game), has mediocre to bad framedata with low damage output per hit compared to a lot of the cast, bad oos, and a bad initial dash (making it much harder to bait opponents with movement).
His grenade egg is a great tool, but has a timer of about 2.5 seconds, so his mix ups with it are limited compared to other characters with item play like Snake, diddy, ROB, etc. Egg shot is a great lingering hitbox to make their ledge trapping pretty annoying for some characters, but again, takes a while to come our, and while breegull blaster has cool combos and techs, it takes a lot of work to do them consistently beyond a couple shots in uair or nair or BB>Utilt, and it's still abysmal damage overall. He does have some nice disjointed pokes with ftilt, dtilt, and bair though, and fair is pretty strong and good in neutral with decent shield pressure with good spacing.
In short,
is (imo) a mid tier that is just a bit undertuned in that he has a lot of "almost" tools that are just short from allowing them to truly shine in a high or top level meta. They either get stuffed out too easily because of the poor airspeed and/or slow start up frames, or whiff-punished too easily because of high cooldown (i.e., wonderwing, usmash) and/or landing lag (nair, bair). These aren't good things to be exploited by in a meta full of high mobility swordies or characters with obscene punish games like
.
I sometimes wonder if, without covid causing the world to go online for a year or more, BK may have had a shot at getting the QoL changes or buffs that would make them a reasonably viable character that still needs a secondary for their atrocious matchups, but oh well.