Answer:
   
   For traditional Torah scholars, the end of each era is marked by a
   book that gets accepted by the masses as authoritative. This seals the
   acts of that era as a whole as accepted, authoritative p'sak.
   Therefore, any ruling by those who live after this era must be
   supported by an opinion of that era.
   
   The first such book (and the first written book of the Oral law) is
   the [5]Mishna. There are other compilations of the Tanaitic material,
   the [6]Braisos (Baraitot), the [7]Tosefta, and [8]Midrashai Halakha
   (Mekhilta, Sifra, and Sifre), but it is the Mishna that marks the end
   of the Tanaitic era (70-200 CE). It was the Mishna that was accepted
   by the people.
   
   The second is the [9]Babylonian Talmud. The [10]Jerusalem Talmud is
   less authoritative because it was developed for a shorter time than
   the Babylonian Talmud. The Talmud marks the end of the Amoraic era
   (220-500 CE).
   
   The next era was the period of the Sabora'im (500-650CE). At this
   time, the Jewish sages in Persia who were the rabbinic leaders of
   their time. They contributed much to the finishing of [11]Talmud
   Bavli; Jews in this area continued to live in a relatively stable
   environment. In contrast, Jews at this time in Israel were living
   under the oppressive rule of the Byzantines.
   
   There is a Ga'onic era in Jewish history (650-1250CE), but not in
   Jewish law, since there is no book that was accepted as the end of
   that era. At this time, Jews were living in Southern Europe and Asia
   Minor under the often intolerant rule of Christian Kings and clerics.
   Most Jews lived in the Muslim Arab realm (Israel, North Africa,
   Babylonia). Despite periods of persecution, Jewish communal and
   cultural life flowered in this period. The universally recognized
   centers of Jewish life were in Sura and Pumbeditha (Babylonia); The
   heads of these law schools were the Geonim, who were consulted on
   matters of law by Jews throughout the world.
   
   The next such book(s) is the [12]Shulchan Aruch (by R' Caro), the
   authoritative Sephardic resource, and the [13]Mappah (Ramah), which
   has the Ashkenazic rulings when different (Note that both are in the
   same book; see the general reading list). This delineated the period
   of the Rishonim (The First Ones) (1250-1550CE). A Rishon may argue
   with another Rishon, or with a Ga'on (since there is no Halachic
   concept of the Gaonic era), but can only argue with an Amora if he has
   another Amora in his support. He cannot use a Tana that was rejected
   by the Amora'im as support, since that would be overruling a p'sak of
   someone greater in chochmah.
   
   Most Jews in the period of the Rishonim lived in the Mediterranean
   basin or in Western Europe under feudal systems. With the decline of
   both the Muslim and Jewish centers of power in Iraq, there was no
   single place in the world which was a recognized center for deciding
   matters of Jewish law and practice. Consequently, the rabbis
   recognized the need for writing commentaries on the Torah and Talmud
   and for writing law codes that would allow Jews anywhere in the world
   to be able to continue living in the Jewish tradition.
   
   Anyone after the Shulchan Aruch is called an Acharon (The Last Ones)
   (1550CE to present). An Acharon can only disagree with a Rishon when
   he is taking the position of another Rishon. There are strict rules
   for change.
   
   Liberal Jews tend to justify halachic change by ascribing greater
   authority to present generations (or even to individuals) than to past
   generations of sages.