Shabbat: The Shacharit Service
There is somewhat of a contradiction in the format and structure of the Shabbat morning Shacharit. On the one hand, the Talmud teaches that the Shabbat morning amida (Shemoneh Esrei) was made significantly shorter than the weekday shemoneh esrei "in order not to burden the congregation with a lengthy prayer service on Shabbat." On the other hand, Pesukei D'zimra, the preliminary morning prayers consisting largely of sections from Tehillim, is exceptionally longer on Shabbat than it is on weekdays!
Therefore, the question is asked, why did the sages shorten one section of the prayers only to lengthen another? What happened to the objective of ensuring a shorter Shabbat morning service?
One answer to this apparent contradiction is that the prayers that the sages chose to limit or shorten were only those related to personal requests. As the weekday Shemoneh Esrei is full of personal and communal requests and supplications, the sages wanted to distinguish the Shabbat amida from the weekday Shemoneh Esrei by removing all such prayers. According to this approach, the "burden" that the sages wanted to relieve on Shabbat was that of having to recite these supplications as part of the Shabbat amida. So too, having to recall our individual and national problems in the course of the Shabbat prayers could arouse feelings of sadness and distress, which is undesired on Shabbat.
On the other hand, prayers that praise God, such as those that were added to the Pesukei D'zimra, are certainly in keeping with the theme of Shabbat. As a result of these additional prayers, the Shabbat morning service is indeed significantly longer than the weekday service. So too, there are additional insertions to the birchot kriat shema on Shabbat, such as the el adon and la’el asher shavat prayers as well as a number of additional insertions prior to mussaf. Therefore, we see from here that the "lengthy prayer service" that the sages wanted to avoid refers only to the Shemoneh Esrei, and not to any other parts of the service. It is interesting to note that "Uva L'tzion", which is normally recited at Shacharit, is likewise moved to Mincha on Shabbat “in order not to overburden the congregation” with an already lengthy service.
 Berachot 21a; Mishna Berura 268:2.
 Rema, OC 281:1; Mishna Berura 281:3, 4.
 Rivevot Ephraim 7:102.
 Yerushalmi, Shabbat 15:3; Vayikra Rabba 34:16.
 Tanchuma, Vayeira; Sefer Kushiot 192.
 Mishna Berura 292:1; Kolbo 40.