From a cursory glance at both projects, it looks like plotnine doesn't do interactivity, for a start?
My (wildly speculative) guess for the motivations behind this library would be that Kotlin, as a base, is usable in the other JetBrains IDE's, e.g. reusing the same library and grammar when developing in Java or Ruby.
I can think of 3-4 off the top of my head.
Would be curious to hear which ones you're thinking of (since I wouldn't be surprised if the are others, but also am guessing no others hit both points)
Plotnine is the only one another human has told me about, though.