I assume it's just not really built for it. Iron Dome is primarily to intercept rockets. They're hot and follow a predictable trajectory. I'm sure it can intercept UAVs if it's small-planed sized stuff. If it's a swarm of nimble little guys like Ukraine is using - I'd think considerably harder to intercept.

But for whatever it's worth, they're claiming they intercepted the majority of them, but they didn't say specifically by Iron Dome. They say they intercepted them over Iraq and Jordan so probably some other system or aircraft?

More than 200 drones, cruise missiles and ballistic missiles were fired from Iran

Assuming one interceptor per threat, that'd be something like $20-30m in munitions.

Edit: I should have known.

Statement by Secretary of Defense Lloyd J. Austin III on Iran's Strikes Against Israel April 13, 2024
At the direction of President Biden, U.S. forces in the Middle East on April 13 intercepted dozens of missiles and UAVs en route to Israel, launched from Iran, Iraq, Syria and Yemen. Our forces remain postured to protect U.S. troops and partners in the region, provide further support for Israel's defense, and enhance regional stability.

So shot down with something likely more expensive than Iron Dome and with the US paying for it instead of Israel. Though I assume the US probably pays for Iron Dome too...