Not much experience in Rails, but I think that this is because Rails is much more modular than Django which in turn is quite large out of the box and it would not be great for smaller web applications. That's where Flask comes in. Whereas Rails can be used for applications of all sizes and that's why there's no need to learn another framework if you already have used it.

Although I feel that Flask is slowly becoming more popular than Django and contrary to popular belief, it's quite good even for large projects.

I think that the situation with Rails inside the Ruby ecosystem is similar to Express within the Node ecosystem. Highly modular frameworks that are used by almost everyone inside their respective ecosystems and even though there are other alternatives that cover specific niches, these two can cover much more than one single niche.