One factor was huge amounts of free credits for the first year or more for any startup that appeared above-board and bothered to ask properly.

Second, egress data being very expensive with ingress being free has contributed to making them sticky gravity holes.

The free credits... what a WILD time! Just show up to a hackathon booth, ask nicely, and you'd get months/years worth of "startup level" credits. Nothing super powerful - basically the equivalent of a few quad core boxes in a broom closet. But still for "free".