Why a two stroke bike engine is losing compression?

It could be a number of things, like a broken or worn ring, or a leaking head or base gasket. If it happened suddenly, I'd suspect a broken ring. If it happened gradually, I'd suspect it's a wear issue. If it's accompanied by a noisy piston, (sometimes called, "piston slap") then I would suggest it's time for a top-end job. Good Luck