Currently most bills submitted contain a multitude of unrelated and usually hidden riders. These are often pet pork projects of the bill's author, or other quid pro quo spending used to pursuade other lawmakers to vote for it.
This is counterproductive to the process for two reasons - first, if the bill passes there is undebated spending that would not pass if it stood alone, which drives up the deficit.
Secondly, it gives lawmakers an out to vote against a bill that might otherwise gain bipartisan support - "I would have voted for hurricane relief, but not if it includes money for Alaskan fisheries.". Ultimately this punishes those for whom the bill was designed to help.
Each bill must address a single issue, and must succeed or fail on that issue's merits alone.