Well, for the domains Mojo targets, Python is king. So a faster-Python-like language would have more potential audiences. A fast Ruby-like language, not so much, as Ruby was never that special in those domains, or in most places outside web development, and even for that it kind of lost steam in the past 10 years.
Besides people opting for closer to C speed had Rust, Go, Java, Swift, and other options to go to, all with more momentum and support, before going for a yet unproven Ruby clone.
I used to be quite sceptical given how Swift for Tensorflow went, however since NVidia decided to partner with Modular, alongside their ongoing CUDA JIT bindings for Python, I think Mojo might actually work out.
Apparently it had Google's money backing, for what it is worth.
I never believed into it, because Swift is as relevant as Objective-C outside NeXT/Apple's platforms, and not the kind of programming language that the research community cares about.
Besides people opting for closer to C speed had Rust, Go, Java, Swift, and other options to go to, all with more momentum and support, before going for a yet unproven Ruby clone.