Ruby's grammar is objectively more complex than Python's. People generally get stuck on syntax issues when they begin learning programming. Python's significantly simpler grammar, simpler and fewer basic building blocks, and historically "only one way to do things" philosophy makes it easier to pick up.

Ruby's conventional control flow also doesn't translate well to lower level languages; Enumerable pretty much replaces all sorts of loops, functions/methods tend to be chained instead of wrapped (math style), implicit returns, and perlisms, all make Ruby more confusing for a first timer language.