cPanel support Ruby on Rails, but usually we've found that the average Ruby user uses more resources than the average PHP user.
I imagine that a Ruby account would, therefore, cost more.
If you are talking about running Ruby on a VPS without a control panel, any competent web host should be able to configure this for you, for a price.
As Ruby is not as popular as PHP not everyone will install it, after all the more things installed on a server the more maintenance that server will need to maintain it.
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