It's "just JavaScript." Every time I context switch between Python on the backend and JavaScript on the frontend I waste stupid amounts of time making silly syntax errors — semicolons in my Python, missing braces in my JavaScript, etc. Some days I might switch a dozen or more times, and it really feels like I'm wasting brain cycles swapping in and out my language knowledge. Staying in a single language feels faster.
A+++ WebDev -- would hire again!