The Trump administration issued an order to all its current and former members not to comply with subpoenas from the House, citing executive privilege. This left all of the best potential witnesses in a serious double-bind: if they obeyed the House subpoenas they would be guilty of violating an executive order (which could be construed as treason in some cases); if they refused the House subpoena they could be subject to contempt charges and jail time. That is the heart of the 'obstruction' article of impeachment: that Trump deliberately prevented witnesses from testifying by creating a threat of serious legal consequences.
This left the House with an unpleasant choice:
- Pursue the matter through the legal system, which would mean a winding course through the Supreme Court, and no possibility of a ruling prior to the Court session in June of 2020. This would drag out the impeachment process almost to the next election cycle before witnesses could be called.
- Go ahead with the limited evidence and testimony available, hoping that the Senate would see both the core of the case and the efforts at obstruction, and demand further witnesses and information on its own watch.
Neither was a good option. Delaying the case until high summer would turn it into even more of a political quagmire — imagine trying to impeach a president in the middle of an election season — and would leave Trump in a position to do more damage while the legal battles ground out. Submitting the case early, on limited evidence, against an openly hostile Senate majority, was unlikely to be successful. I like to imagine they chose the latter as an act of moral integrity — a 'damn the torpedos; full speed ahead' moment — but I don't know enough about their inner-circle reasoning to state that as a fact.
In any case, the Trump administration's actions precluded calling witnesses without a lengthy court battle. A few people braved the legal consequences and testified anyway, which is where we received the information we have, but most of Trump's closest allies refused to participate.