Andrew McCarthy explains:

Of the seven subpoenas reportedly issued by the House [Intelligence] committee, four are said to involve the Russia aspect of the probe. Fox News reports that these subpoenas substantially duplicate information already sought by the Senate committee, and are directed at alleged collusion, at longtime Trump personal attorney Michael Cohen…, and at retired General Michael Flynn, Trump’s original national-security adviser…

The three subpoenas relating to the unmasking aspect of the investigation are directed to the CIA, the National Security Agency, and the Federal Bureau of Investigation. As the Fox report elaborates, the House committee has claimed that, while the NSA has been cooperative in making voluntary disclosures, the CIA and FBI have been slow to respond. Hence, the subpoena.

As I have previously explained, the CIA, NSA, and FBI are the investigative components of the 17-component “community” of intelligence agencies. It is those three agencies that collect raw intelligence and that make decisions about what identities should be unmasked…

Under the rules that apply to foreign-intelligence-collection, there is a presumption against revealing the names of American citizens. But there are significant loopholes: The names may be unmasked if intelligence officials determine that knowing the identity of an American is necessary in order to understand and exploit the intelligence value of the information collected.

Thus, as I’ve also outlined, it is unlikely that any single instance of unmasking would be found to be a violation of law ”” and, indeed, it would not violate any penal statute (it would violate court-ordered “minimization” procedures). Nevertheless, were a pattern of unmasking established, divorced from any proper foreign-intelligence purpose, that would be a profound abuse of power in the nature of a “high crime and misdemeanor” ”” the Constitution’s predicate for impeachment.

The trouble with investigations like this is that they are of necessity slow. There is no way around that, but it means that to most people it’s a big yawn, if they manage to follow it at all. It also means that any action comes way too late to stop the thing from happening till the damage is already done (or the advantage gained, depending on what side you’re on).

Another related problem is that these things tend to be very legalistic and complex. That also makes them hard to follow and instrinsically somewhat boring to most people. It’s much more fun, and easier, to talk about Trump’s “covfefe.”