Why? Azure egress bandwidth is too expensive for a Tor relay, even for a well-paid software engineer like me. I have ~1.5 Gbps Tor bandwidth. If I were to put that on Azure, I would be paying a mortgage worth of bandwidth fees. Same can apply to AWS and Google Cloud.
However, Big Clouds charge more for bandwidth because (a) they're a leader and (b) they have paid peering deals with Big Telecom (e.g. AT&T, Deutsche Telekom, Telefonica, etc.). Smaller hosts like Hetzner don't peer with those ISPs and rely on "transit", but that also means slightly worse performance to Big Telecom.
In short, big clouds can use their market power and advanced technology to command a premium, whereas smaller clouds have to win over customers with lower prices to make up.
Tor relays are ill-suited for Big Clouds, but then your 5000-node Kubernetes cluster is ill-suited for OVH or Hetzner.
I do use the free Azure credit for Tor Bridges, however.