The Maror (bitter herb) was an adjunct to the Passover sacrifice, rather than a mitzvah in and of itself. By rabbinic enactment, it was instituted to be eaten at the Passover Seder, in memory of the bitterness of slavery, as well as to inculcate the memory and feeling of the original, Temple era Passover. Just as the Talmud enumerates five types of grain that are acceptable for matzah, it also enumerates five kinds of bitter herbs that are acceptable as maror. However, unlike the case of the grains, where all others are excluded, there is a debate among rabbis if the five herbs are exclusive, or merely examples of what is acceptable. I am of the view that only the five that are enumerated are OK, many disagree with that stance. Unlike in the case of the grains, where we are uncertain about three , we do know which the five bitters are, as the rabbis in Babylon immediately after the Talmudic era left us an Arabic translation of the list. The problems arose as the Jews settled in Europe, where the flora and fauna are different from those in the Middle East. Folklore stepped in and connected various species with those in the Talmud. This has caused many debates, some lasting to this very day. For example, one of the grains mentioned for matzah is "shibolet shu'al". Since at least the tenth century, this has been interpreted as "oats". But, as the Talmud states that shibolet shu'al is a variety of barley, and oats are as close to barley as I am to an artichoke, many oppose it's use for matzah.This was already written about in the 11th century. One famous Jerusalem rabbi of the late 20th century came to this conclusion. A colleague chastised him for breaking with tradition, upon which he reversed himself. As the Jews migrated further North in Europe, the herbs mentioned in the Talmud, all of which are leafy vegetables, simply were not available yet in early Spring. Horseradish was substituted, although unknown in the Talmud, or even in the Middle East until modern times. According to the view that any bitter vegetable is OK when the specific ones are unavailable, that would be fine. Those who didn't follow that view, argued that horseradish was,in fact, one or another of the herbs mentioned. This was fanciful (the Jerusalem Talmud even indicates that all the herbs are either lettuce or related plants). It should be noted that in the U.S., most traditional (but not knowledgeable) Jews use prepared (pickled) horseradish. This is clearly not acceptable according to any authorities. The Maror must be fresh; not pickled, not cooked. This has become another "tradition" that is contrary to halachah. During all my years in the congregational rabbinate, conducting communal sedarim, most people would not accept lettuce, or at least fresh ground horseradish, for Maror. It was a losing battle. 
In some communities only lettuce is used (this is my practice). Some of those insist on one particular variety or another (I don't). Some wrap fresh horseradish in lettuce leaves. Some use endive, which IS one of the five. Some use wild lettuce, which is almost unbearably bitter, albeit one of the five. But why lettuce? It isn't really bitter. It is clear that the Talmud prefers lettuce over the other options. This would seem strange. That will be the topic of my next post.