Why does it need a sequel?
It's not why, but rather,
why not?
Nothing really "needs" a sequel. Metal Gear Solid didn't need a sequel, it was a great game on its own. Same with Silent Hill. Or Suikoden. Or Gears of War. Or Mass Effect. Or Red Dead Redemption. Or Dishonored. Or Bayonetta. The list goes on and on. But all their sequels were widely acclaimed games on their own and highly regarded by fans.
No reason why Bloodborne shouldn't be among those. And as an exclusive IP it's honestly baffling that Sony hasn't capitalized on that more.
(Side note: I actually hate some of the games above, I didn't just pick my favourites to make a point :P)
The setting isn't good, the performance isn't good, and the music is a step down from the previous entries.
I know about opinions and all but this is wild, not gonna lie. The soundtrack is by far my favourite in the Soulsborne series, the setting is amazing and widely regarded as the best too, and as for performance, while it has some problems, it still was better overall than Demon's Souls as well as Dark Souls 1 on PS3.
I would reluctantly agree that Bloodborne does a few things worse than Demon's/Dark 1. The online, particularly the PvP, is worse (and Dark 1's PvP was pretty bad too, but Bloodborne had major deal-breaking problems with invasions such as the Sinister Bell mechanic and how blood vials/bullets couldn't be easily restocked), not being able to warp from the lamps, and the lack of build variety.
However, it improved on so much more, and was overall such an incredible game that I wouldn't blame anyone for overlooking those things.