The omnibus appropriations bill funding the U.S. government for the rest of fiscal year 2014 is H.R. 3547. The original title and substance of that measure was "A bill to extend Government liability, subject to appropriation, for certain third-party claims arising from commercial space launches." So why is this insurance claims bill the vehicle for the big money bill?
Short answer: because it makes passage easier. Longer answer: because it avoids a filibuster in the Senate and amendments in the House. In other words, it allows the compromise package to avoid delays or dismantlement.
The traditional way to settle differences between chambers is a conference committee. Even there, the way the conferees report their action, often in terms of receding from amendments by one house and so forth, limits what can be done to the resulting package. Another way to resolve differences is legislative ping pong, sending the "papers" on the measure back and forth until both houses pass the same language.
In this case, the House had passed H.R. 3547 on December 2, 2013. Ten days later, the Senate passed the bill with a small amendment increasing the extension of the liability law and sending it back to the House.
When the appropriators reached their package, they chose to attach it as an amendment to H.R. 3547, thereby gaining the protections mentioned earlier. CRS has a good paper explaining some of this arcana. The lesson here is that different rules lead to different outcomes, and good lawmakers know how to play by the rules.