Passage: The Democratic Party dominated the House of Representatives during the administration of President Franklin D. Roosevelt (1933–1945), often winning over two-thirds of the seats. Both Democrats and Republicans were in power at various times during the next decade. The Democratic Party maintained control of the House from 1955 until 1995. In the mid-1970s, there were major reforms of the House, strengthening the power of sub-committees at the expense of committee chairs and allowing party leaders to nominate committee chairs. These actions were taken to undermine the seniority system, and to reduce the ability of a small number of senior members to obstruct legislation they did not favor. There was also a shift from the 1990s to greater control of the legislative program by the majority party; the power of party leaders (especially the speaker) grew considerably.

Question: Why were there major reforms of the house in the mid 1970s?
undermine the seniority system, and to reduce the ability of a small number of senior members to obstruct legislation