Subject: Re: U.S. Treasury Sanction Hamas
Why weren't we doing this [imposing sanctions on terrorists] already?

Apparently we were, for quite a while now, but I would suspect that these new sanctions are based on recent intelligence info. From https://home.treasury.gov/news...

Today's designations are part of a continuous effort by the United States to root out Hamas's sources of revenue in the West Bank and Gaza and across the region and is taken in close coordination with regional partners and allies. To date, Treasury has targeted nearly 1,000 individuals and entities connected to terrorism and terrorist financing by the Iranian regime and its proxies, including Hamas, Hizballah, and other Iran-aligned terrorist groups in the region.