First, many members of Congress now depend less on the party’s financial and other support. This was manifest in the highly public pledge of the Koch brothers’ network to support Republicans who bucked the party leadership. But it is not just these big funders who have changed the landscape. The communications revolution has enabled individual members of Congress to connect effectively with small donors throughout the country. Small donors (like other individual donors) tend to be the most ideologically polarized source of money in politics, and they further empower the extreme wings of the parties to stand up against more centrist leadership.
Second, committee assignments seem to matter less than at some points in the past, which also diminishes the leverage of party leaders. Committee assignments are less necessary for name recognition and fundraising. Committees are less important as centers of policymaking, as was obviously the case with the AHCA. Because committees were not particularly important designers of that bill, individual members had no significant role in shaping it.
Third, the end of earmarks has taken away a tool that party leaders can use to entice members to support legislation that the leadership views as a priority.