It boils down to two things: input lag and pixel density. Input lag is the time between you pressing a button and something happening on the screen. A telly isn't designed for rapidly changing inputs, it's one input (usually 30fps) which stays constant, aka movies and tv shows. Monitors are designed to handle this much more efficiently, hence their higher refresh rates.
Pixel density is how blurry the image is. This ks negated by how far you sit away from it though, but close up, a monitor will always be clearer than a telly. Tellies also tend to "blend" pixels to make the image smoother, which looks nice from ghe sofa but up close it just looks ugly.