Why did Hillel advise eating the meat of the Paschal Lamb in this way?

The Torah commands us that on the afternoon before the Pesach holiday, we should roast a goat or lamb kid over a flame and then consume the meat together with the Matzah and the bitter herbs later that night at the holiday meal.

Now each of these foods has its own significance. But it is implied by the wording of the Torah verse that we should eat them all together. The verse reads, “ al Matzot u’merorim yochluhu, ” literally, “you shall eat it [the meat of the Paschal Lamb]  upon  matzot and bitter herbs.” Wishing to be in full compliance with the instructions of the verse, Hillel popularized the custom of actually placing the sacrificial meat on top of the maror and matzah and eating them all together. In fact, according to Hillel, one would not have fulfilled his Paschal obligations unless he consumed the three together.

Now, for the past two thousand years when we have not had a Temple and have been unable to perform the sacrificial rites, we cannot fulfill the Mitzvah of eating the Pesach sacrifice at our Seder. We do however have the ability to do the mitzvah of eating matzah, which is a separate commandment by itself. And although eating maror is really part of the Biblical commandment of eating the sacrificial meat and not a mitzvah of its own – the Sages enacted a rabbinical commandment that we continue to eat the maror as a remembrance of the Biblical command which we cannot observe today.

haggadah Section: Koreich
Source: Ask Moses