Someone else ask a very similar question also :-). Here is my answer:
Jellyfish are not the best swimmers so they drift a lot with the tides and ocean currents. Strong onshore winds will also bring the surface water towards the shore, and with it comes the jellyfish. So while jellyfish are found all over the oceans, we mostly see them near shore when the wind, weather and currents have driven them there. this is also because we humans are mostly near the shore and not on or in the water, so we will see them there if we see them at all :-).