I don't understand why West Ham appointed a manager who has a bad record in previous EPL coaching (when he was Brighton and Chelsea manager) West Ham should have appointed a manager who has a good record to restore their performance if they appointed a manager who is not much different why change managers when currently West Ham is really in a worse condition than last season and I doubt Graham Potter can handle a club that is in a slump.
He's good at Brighton, but flop in Chelsea. He has good stats when managing medium tier club, and why are you doubting it? He has also experience in the Premier League. I think it's reasonable to believe Potter will deliver for Westham.
Are you thinking Westham to repeat the same mistake to gamble with the coach with no PL experience? They have realized they have to found someone who can fit them well, and Potter is the answer.
I just checked in case if I remembered it wrong, he averaged 1.28 points at Brighton. That is not good at all, I am not saying he should have done 2.50 or something, it's Brighton, they were worse than even right now, so it's normal to have bad record, but at least 1.50+ is required to say it would have been good, it's really a bad period for Brighton as well and dude was sacked, why would they fire a manager who was good.
And obviously he was terrible as Chelsea manager as well, and you may ask if he was not good at Brighton, then why would Chelsea get him? Well, simply because of exact same reason why West Ham are getting him now, we have no idea. In the end, managers are limited, they are not as common as players, you have at least 11 players, with bunch of subs, whereas you have just one manager, so teams just want to hire managers who already have experience at good levels.