Simple, LibreELEC heavily patches kernel and ffmpeg for better experience. Patches are slowly getting in kernel but ffmpeg patches will need far more time to be accepted (kernel must first declare this API stable which won't happen anytime soon). You also need Kodi master (e.g. Kodi 19 pre-alpha), because it has a lot of important improvements. Last but not least, only Kodi GBM (e.g no desktop environment) version really works well (zero copy playback) and soon wayland. X11 has no chances to work well.
TL;DR: LibreELEC uses features which are in development and not present in any stable version yet.
BTW, also 4K HEVC videos work well.