Location, the kind of position, perceived company reputation and pay...

I've been on your side of the table for a similar role and had the same experience. Literally 20-30 candidates who can't answer basic questions for every 1 who can (and usually the 1 is "just ok").

I think it's just due to the particular pool your company is dealing with. Maybe there is a surplus, maybe there isn't, but you're not able to judge that objectively from your vantage point.