Angelo Codevilla addresses that question in an interesting post at American Greatness:

Casting aside “regular order” was essential to the rise of the unaccountable administrative state and the near-sovereignty of party leaders, lobbyists, and bureaucrats.

[Under the system of regular order that held sway from the 1790s until the end of the 20th century] bills introduced in House or Senate would be sent to the relevant committee, and thence to the proper sub-committee. The ones thought worthy—including those funding the federal government’s operations—would be the subject of public hearings. …

[Following subcommittee and full committee hearings] the full committee’s “mark up” [would] be scheduled for action on the House or Senate floor.

Just to get to this point, every element of every bill had to be exposed to public scrutiny. Senators or congressmen on the committees offered amendments and had to vote on the record for each part of the bill. On the House floor, amendments would be limited. But in the Senate, there could be—and often were—“amendments by way of substitution.” By the time the “yeas and nays” were tallied on the final bill, just about all members had had as much of a crack at it as they wanted. The final product would be the result of countless compromises “on the record.” …

Senators and congressmen abandoned regular order because it hinders their craving for power and flight from responsibility. Voters elect them to vote accountably on important matters. But since such matters are almost inevitably divisive, they do their utmost to avoid voting on them. …

Regular order had forced them to be … responsible to the voters. They prefer to be safe, indistinguishable, comfortable among courtiers.

Regular order’s death came about in this way. For over a century, congressmen and senators’ procrastination had pressed legislative business into the last weeks before the end of congressional sessions. Members had noted that they could slip items into bills in frenzied times, which would not have survived regular order’s scrutiny. In the 1970s, some committees started to procrastinate on purpose, so that the end of the government’s fiscal year would come without an appropriation for one or more department of government. The Appropriations Committee would then prepare a “continuing resolution” to substitute for the uncompleted appropriations. These were supposed to just “keep thing going next year as in the previous year,.” … But it was never that simple: from the beginning, these CRs always had riders. The more influence you had, the more you could slip into the CR. …

The Democrats’ control of the Senate and Harry Reid’s control of the Senate following the 2006 elections changed American government radically. In fiscal years 2007 and 2008, by preventing any committee from producing any appropriation bill for any government agency, Reid made sure that all of the U.S government’s business would be compressed into one CR, the contents of which would be negotiated strictly between himself and President George W. Bush, whom Reid had over the proverbial barrel. Between 2009 and 2015, the same tactic yielded a federal government that was the “cosa nostra” of Harry Reid, Barack Obama, and Nancy Pelosi. …

[W]hen John Boehner replaced Pelosi as Speaker of the House, his vow to enforce “regular order” amounted to nothing. Same for Paul Ryan. Senate Majority Leader Mitch McConnell since 2015 has been a Harry Reid wannabe—minus the competence, plus the pretense. …

Merely holding the line against the establishment’s continuously mounting claims on the rest of America—never mind reversing them—will require re-involving the American people in their own business. That means restoring Congress as the American people’s primary representative institution. Making Congress work according to regular order, and only through regular order, is a prerequisite.