Why aren't there subpoenas issued yet?

[The Velveteen Ocelot]

1. Tactical and strategic reasons as well as legal and political ones.

Generally speaking they will ask people to appear voluntarily, and subpoena them only if they refuse, because it's better to have a cooperative witness than one who was dragged in involuntarily. Either way, witnesses are legally obligated to testify truthfully, but you're likely to get more complete answers from someone who is testifying without being subpoenaed. And the committees can somewhat avoid accusations that they are being unnecessarily confrontational. I also assume they are waiting to see what Barr does next and what else is disclosed, and if Barr continues to stonewall they will have no choice but to subpoena some people. I think Mueller will appear without a subpoena if asked. Sometimes a "friendly" subpoena is issued just so a willing witness' butt is covered and he won't be accused of favoring the committee. It's a lot more complicated than just throwing out a bunch of subpoenas to whomever.