I can't say I'm an expert but i have a "Not bad information's" about it.
1-if one Service fall i start using the Other Example:I have 2 Internet subscription to my home and if one fall and lost connection i start Using the Other(not the same ISP) they are in contract with one Server Service can't they contract another(it might be expensive or legally they can't I don't know what are the contract terms or Itch.io budget).
2-WIth the technology we have isn't easer than before the internet and Computer World are evolving by time and the market offer many things hardware/Software with different prices as for maintain their own Data-center they can hire an IT that specialized in those stuff.
3-well i compared it to GameJolt witch should be an equal competitor i use both itch and gamejolt daily and i didn't notice gamejolt getting down there are sometime that you feel that the Website no responsive or slow but not down while itch it can suddenly be down for 1+ hour.