We left Egypt very quickly and suddenly. Therefore, we cook the Korban Pesach in the quickest possible way, which is roasting. We eat the Korban Pesach like noblemen. Wealthy people roast their meat. Although it shrinks, it is very tasty.
Poor people boil meat.
Although it does not taste as good, it absorbs water and expands, so there is more meat to eat. But a rich man doesn't worry about having enough to eat. Tonight, we eat like rich men. Now, Shmully, when we eat our Korban Pesach, make sure you do not leave any leftovers. If any meat is leftover the next morning, we must burn it.