Why does Congress make laws that expire?
This year we saw a lot of drama around whether or not Congress would renew an expiring ruling onthe interest rates for student loans. Then a line from an article on the recent Colorado Shooting caught my eye:
If we at one point thought it made sense to ban assault weapons for private ownership, why was that ban part of a law set to expire? why not make laws and then, when and if someone decides they no longer make sense, let them repeal them or make new laws? Expiration dates on sseem rather arbitrary and therefore nonsensical.
Can anyone explain this to me?
The AR-15 rife carried by Holmes, a civilian semi-automatic version of the military M-16, would have been defined as a “semiautomatic assault weapon” under the Violent Crime Control and Law Enforcement Act of 1994 — which expired in 2004. “The type of ammunition magazine Holmes is accused of using was banned for new production under the old federal assault weapon ban.” Though once it expired, “gun manufacturers flooded the market with the type of high-capacity magazines Holmes used Friday.”
If we at one point thought it made sense to ban assault weapons for private ownership, why was that ban part of a law set to expire? why not make laws and then, when and if someone decides they no longer make sense, let them repeal them or make new laws? Expiration dates on sseem rather arbitrary and therefore nonsensical.
Can anyone explain this to me?

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Generally, but if the law has changed, a court may decide that a precedent no longer applies, and ignore it. On appeal, higher courts will decide if the lower court was right to do so. The legal system has always had to deal with the laws changing underneath it; it's not an unusual thing.
I don't think that's a problem, though; the expiration dates aren't a secret, and companies and people can plan for them, as can judges and bureaucrats.
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Hmmm...it appears I'm not the first one to suggest this.