מרור זה שאנו אוכלים על שום שמררו המצרים את חיי אבותינו במצרים
The reason for the central place in the Seder of Matzah and Pesach is understood since they both allude to the redemption. But it is not clearly understood how Marror, which alludes to the bitterness and affliction that occurred in Egypt, could also allude to the redemption.
However, we know from Parshas Lech Lecha that the exile in Egypt was supposed to last 400 years. However, the difficulty and increased bitterness that actually did occur led to a reduction of the exile by 190 years (only 210 years as explained by Rashi Shmos 12,41). Thus, Marror is fitting to be included as one of the main three components since it does allude to the difficulty of the exile which contributed to the early redemption.