On every Yom Tob, we take out two Sifreh Torah, and we read from the second Torah the Pesukim that describe that holiday’s special Musaf sacrifice. Likewise, when Rosh Hodesh falls in Shabbat, we take out a second Sefer Torah and read the Pesukim that speak about the Musaf sacrifice offered on Rosh Hodesh.
The question arises, why do we not read every Shabbat the Pesukim that describe the Musaf sacrifice of Shabbat? Just as on Rosh Hodesh and holidays we make a point of reading the Pesukim that describe the day’s Musaf sacrifice, we should, seemingly, read every Shabbat the Pesukim that describe the Musaf sacrifice for Shabbat. We should take a second Sefer Torah, and after completing the reading of the weekly portion, read from the second scroll as the Maftir the verses that describe the special Shabbat sacrifice.
Three main answers have been given for why we do not read the Pesukim of the Musaf sacrifice every Shabbat. First, we read the Pesukim of the Musaf sacrifice so that we can be considered as though we, ourselves, bring the sacrifice and thereby earn atonement. This rationale does not apply to the Musaf sacrifice of Shabbat, which, unlike the other Musaf sacrifices, does not include a Hatat (sin-offering). This sacrifice was not brought for atonement purposes, and thus there is no reason for us to read it every Shabbat.
Secondly, there is a general rule that the Haftara reading from the Prophets must relate in some way to the content of the Maftir reading. In light of his rule, if we would read as the Maftir every Shabbat the Pesukim that describe the Musaf offering, then we would have to read the same Haftara each and every week, which would certainly not be desirable.
The third – and perhaps simplest – answer is that the section in the Torah which describes the Musaf offering on Shabbat consists of only two Pesukim. We never read an Aliya that contains fewer than three Pesukim, and so we cannot read this section as the Maftir on Shabbat. It would be inappropriate to being the Maftir reading several verses earlier, in the section dealing with the daily Tamid sacrifice, because this section is not at all relevant to the occasion of Shabbat, and we do not add irrelevant verses to the Torah reading.
These three reasons are all alluded to in the second of the two Pesukim that speak of the Shabbat Musaf sacrifice: "Olat Shabbat Be’Shabbato Al Olat Ha’tamid Ve’niskah" ("It is the burnt-offering brought every Shabbat, in addition to the daily burnt-offering and its libation"). The first word – "Olat" – indicates that the sacrifice brought on Shabbat was an Ola (burnt-offering), and did not include a Hatat, such that there is no atonement earned through this Torah reading. The next phrase – "Shabbat Be’Shabbato" – alludes to the fact that reading these Pesukim as Maftir would necessitate reading the exact same Haftara each week. Finally, "Al Olat Ha’tamid" – the previous section deals with the Olat Ha’tamid, which does not directly relate to Shabbat, and it would thus be inappropriate to add verses from that section so we can read the Pesukim of the Shabbat sacrifice.