It is a rational decision (though from a science point of view I would have wished for another decision). Would you want to pay a very high premium to prevent (perhaps) a disaster with an extremely low probability?

We know a lot about the frequency of these events, may they be Tunguska size events or 90% killing of lifeforms events. Those with more than a local disaster potential are extremely rare.

In 50 years, there might be another more advanced technology to deal with such threats. To pay an enormous sum of money just to reduce that risk for the short time before we can deal even better with it is not very rational.

But that argumentation (prevent disaster) is exactly what I would try to sell politicians who can grant money (or not) if I was a scientist interested in astronomy. I wouldn't believe it myself but I would hope the politicians would.

Wolfgang