The mishna (Megillah 21a) outlines the number of individuals called to the Torah throughout the year. Although one cannot reduce the number of required aliyos, occasionally additional aliyos are permitted. Rashi explains that additional aliyos unnecessarily prolong the tefillah, causing undue hardship to those who work. Accordingly, on Shabbos, Yom Kippur, and yom tov, when melacha is forbidden, hosafos are permitted.
The Ran, however, maintains that the number of aliyos reflect the level of the kedushas hayom, the sanctity of the day. To underscore the heightened sanctity of yom tov above chol hamoed, the chachamim insist that on yom tov an extra person is called to the Torah. If one would add aliyos on chol hamoed, it would appear as though one is inappropriately equating yom tov and chol hamoed. If so, hosafos would only be allowed on Shabbos, the pinnacle of kedushas hayom.
Although hosafos are generally permitted on Shabbos, the mishna (Megillah 31a) teaches, "ein mafsikin b'klalos, ela echad koreh es kulo - we may not interrupt the reading of the curses, rather one person is called and reads the entire passage."
The gemara (31a) which limits the teaching to the tochacha in parshas Bechukosai offers two explanations of this mishna. According to Rav Chiya bar Gamda amar Rabi Assi our mishna reflects the passuk (Mishle 3), "mussar Hashem b'ni al timmas - the admonishment of Hashem, my son, do not disdain," which underscores the inappropriateness of interrupting mussar Hashem. Reish Lakish, however, maintains that the mishna is based on the general principle, "ein omrim beracha al hapuranus - we do not recite a bracha on punishment." If an aliyah was added in the middle of the tochacha, the new oleh would be reciting a bracha on a davar ra.
The Taz (Orach Chaim 428) raises an obvious difficulty. Although the view of Reish Lakish conforms to our common practice where each oleh recites his own brachos, the custom in the time of the mishna was that only the first and last oleh recite brachos. If so, even if we were to add an oleh in the middle of the tochacha, the new oleh would not recite an additional bracha. How, then, can Reish Lakish explain the mishna in this way?
The Taz answers that although in the time of the mishna each oleh relied on the first bracha of the kohein and the concluding bracha of the acharon, each oleh could recite his own bracha. Indeed if an individual called to the Torah did not hear the initial bracha, he would be obligated to recite his own bracha. Therefore the concern of Reish Lakish is applicable even in earlier times.
The Nachalas Yaakov (peirush to maseches Sofirm, perek 12) based on the maseches Sofirm and Yerushalmi suggests a novel answer. Although generally only one set of brachos was recited for the entire krias haTorah, this was not true regarding the reading of the tochacha. The gemara (Megillah 31b) explains that the reading of the berachos u'klalos this Shabbos is a special enactment of Ezra. "Ezra tikein lahem l'Yisroel she'yehu korin klalos sheb'toras kohanim kodem atzeres v'sheb'mishna Torah kodem Rosh Hashana - Erza decreed that we should read the tochacha of Vayikra prior to Shavuos and the tochacha of Devarim before Rosh Hashana". As an independent takana of krias haTorah separate brachos were recited even in the time of the mishna. Therefore Reish Lakish explains that hosafos are forbidden.
The gemara teaches that we read the tochacha at this time to emphasize that "Atzeres nami Rosh Hashana he d'tnan u'b'Atzeres al peiros hailan - Shavuos is also the beginning of the year, for on Shavuos we are judged concerning the fruit of the trees." May we merit the bracha of "Abayey v'e'teima Reish Lakish - kdai shetichle hashana v'kilaloseha" - Shavuos should mark an end to all curse, tragedy and suffering.