Prior to 1920, there wasn't a consistent process for drafting the annual spending bills. Originally the Ways and Means Committee would write a single appropriations bill to fund the entire government for the year (so technically an omnibus in the sense that it funded everything, but on a much smaller scale), but as federal spending increased, the workload became more than Ways and Means could deal with in addition to their other responsibilities. The House created a dedicated Appropriations Committee in 1865 with responsibility for crafting the spending bills, but you still get some spending bills coming from the relevant authorizing committees. The spending bills in this time period were more issue focused than the agency focus we see today. You'd have one bill that funded salaries and working expenses for all of the agencies, one bill that funded all projects related to rivers and harbors, one bill that funded all military activities, and my personal favorite - the Sundry Civil bill, which funded all of the domestic programs that didn't fit neatly into another bill.

In 1920 the House changed its rules so that all funding bills have to come from the Appropriations Committees and in combination with the Budget and Accounting Act of 1921 (which established a formalized budget process that begins with the President submitting a budget request to Congress), the basic structure of the modern appropriations process is established. Under the "textbook" appropriations process, each of the (currently 12) subcommittees of the House Appropriations Committee are responsible for drafting a single bill providing money for all of the agencies and programs under their jurisdiction. These are the "regular appropriations" bills. Sometimes the jurisdictions are relatively focused - the Defense subcommittee allocates funding to the Departments of Army, Navy, Air Force, and other Defense agencies (excluding military construction) and the intelligence agencies. Other subcommittees have pretty broad jurisdiction - Labor-HHS-Education allocates money to the Departments of Labor, (most of) HHS, and Education and a host of smaller entities from the Corporation for Public Broadcasting to the Social Security Administration. The number of subcommittees has remained relatively stable over time, varying from 10 to 15, with new agencies and programs folded into the existing subcommittees. With each subcommittee writing a single bill, we have naturally seen each appropriations bill expand to cover an increasing number of programs. Funding for the Department of Treasury was a stand alone bill in 1920, was combined with the Post Office in 1924, expanded to include "general government" funding in 1962, was merged with the Transportation appropriations bill in 2003, and was folded into the Financial Services and General Government bill in 2007.

The bundling within the subcommittees is on a smaller scale than what we're seeing with the modern omnibus spending bills, but since each agency is funded under its own title, they're very easy to combine. A bill is considered an omnibus appropriations bill when it bundles together several (or all) of the regular appropriations bills that are supposed to come out of the subcommittees. The first omnibus appropriations bill was passed in 1950 as something of an experiment to try and streamline the process. The thought was that combining all of the appropriations into a single bill would allow members to better evaluate where the money was going and provide "greater care and attention in the appropriations process" (Nelson, 1953). In practice, the members found the resulting bill to be unwieldy and diminished congressional influence over spending so it ended up being a one year experiment.

In the 80s and 90s, Congress occasionally turned to the omnibus appropriations bill as a matter of necessity. Typically the House and Senate appropriations subcommittees start working on their draft bills for the next fiscal year in the spring, shortly after a budget resolution is adopted. The bills are considered on the floor in the summer, the House and Senate go to conference to work out any disagreements, and the bill is signed by the president. All of this has to occur before the end of the fiscal year on September 30th, or the agencies run out of money and shut down. In practice, Congress passes continuing resolutions (CRs) to keep funding any agencies for which they haven't passed the necessary appropriations to buy themselves the time to work out an agreement. If they've failed to pass a substantial number of bills by the end of the fiscal year, an omnibus allows them to consider all of the remaining bills at once. Compromise is a factor - everyone gets things they like and dislike in an omnibus - but that's true of most of the regular appropriations bills as well. The usage of the omnibus is mostly a matter of timing. You're most likely to see an omnibus spending bill at the end of the year when Congress wants to leave town and has run out of time to consider the bills individually.

After the 1950 experiment, we see something resembling an omnibus spending bill again in 1985 (although it wasn't referred to as an omnibus at the time.) The FY86 budget process was particularly contentious. Democrats in the House objected to the president's proposed spending cuts, while Republicans in the Senate were unhappy with the overall deficit. They spent months fighting over a budget resolution before they even got to the appropriations bills so there was really no chance they would be done in time. Add to this an ongoing debate about long-term budget deficits and the debate over the Balanced Budget and Deficit Control Act of 1985. Of the 13 regular appropriations bills for FY86, none were passed by the September 30th deadline. Congress had to pass four continuing resolutions over the next three months, during which time they finished six of the regular appropriations bills. The final seven bills were never signed into law. Instead a single continuing resolution was passed in December which continued funding all of the remaining agencies (with some modifications) through the end of FY86. A similar process plays out in the next two years.

In 1996 we see an omnibus appropriations bill used to resolve the ongoing conflict between Clinton and the Republican Congress after two government shutdowns. The Omnibus Consolidated Recissions and Appropriations Act of 1996 included funding for all programs under the Departments of Commerce, Justice, State, Interior, Labor, Health and Human Service, Education, Veterans Affairs, and Housing and Urban Development. Again, the primary reason for bundling all of these bills together is timing. By the time Congress was considering the bills for these agencies, it was already halfway into the fiscal year they were funding. Congress was supposed to be working on the FY97 appropriations bills by then, not still fighting it out over the FY96 bills. Unsurprisingly, Congress did not finish the appropriations bills in time for FY07 either. Since 1996 we've seen consolidated or omnibus spending bills in 20 of the last 25 years. In some years, a consolidated appropriations bill may only include 2 or 3 appropriations bills that they ran out of time to consider, while in others the budget process has broken down and an omnibus is needed to continue funding most of the government.