Probably a few reasons for this. I’m not a ruby dev so take this with a grain of salt.
Ruby doesn’t have a lot to offer beyond languages like Python or Go without its companion web development framework Rails. Ruby on Rails was good for its time (~2012 -> 2015 era was peak), but there are more mature, stable, and widely adopted frameworks available in other languages. RoR touted speed to develop as a feature, but you can do things plenty fast with the aforementioned languages too. On the flip side, rails apps are notoriously slow to boot. I think this became a problem with cloud native infrastructure. For example, Kubernetes likes to spin up services very quickly, and can be painful to work with if that’s not an option (experienced this with Java apps too for that matter). As self hosting on bare metal went by the wayside, so too did interest in developing new apps on rails, imho.