Or the high consequence of an individual miss. Not that I disagree that Patriot took a long time to evolve to effectiveness, but it's probably more complicated. Take for example the Phalanx CIWS, which fires hundreds of rounds at each target, even though theoretically a single round should be enough, because missing a single incoming target would be very, very bad.
Why? Just because it's expensive?
The resultant tumbling made them highly inaccurate as to target, but it has been suggested it also made them much harder to shoot down.
Sources: https://fas.org/nuke/guide/iraq/missile/al_hussein.htm, https://en.wikipedia.org/wiki/Al_Hussein_(missile)