Why Canvas Render Mode "Screen Space - Overlay" doesn't support Particle System?

I'm trying to make a Particle System VFx in my UI button and I tried for several hours to discover what was wrong..... I used the canvas Render Mode in Screen Space - Overlay and no way.
But when I changed the canvas to Screen Space - Camera it worked perfectly.
Ok, my problem is solved, but I still don't know why?
is there an explanation ?



thanks
Patrick Carvalho