Which makes me wonder if this is more of a calculated move.
A) Establishment Democrats wanted no part of an impeachment trial during a Trump 1st term. Past experience has not been good to the impeaching party. After the phone call to the Ukraine, not impeaching Trump was likely going to cause more harm with their base and affect turnout in 2020. Many would see those Democrats as complicit and many might even see primary challenges if they didn't do something.
B) This impeachment isn't about removing Trump. The Democrats want to do that in the election next year and think they have a very good chance at doing it (yeah, 2016 blah blah blah). So the goal was to show how guilty Trump was but do that as quickly as possible as to not interfere with Primary season and election season. Going to court over Subpoena's pushes the impeachment trial back to the spring at the earliest and for a ruling in the Supreme Court that would not likely go the Democrats way anyway (which is another issue that needs to be taken up).
C) The goal is to put these Republican Senators in the position they are in right now, having to make absurd defenses of behavior that is absolutely ridiculous, with the hope that it costs the GOP seats in the Senate next year. The only issue is GOP voters have shown an astounding disregard for law and order and political norms so there is a good chance that even as dumb as all these Senators look, they won't care. How long is it before some of these Republicans just come out and say it? "Yeah, this really isn't an appropriate action and I would never stand by and let a Democrat do this, but we really need to keep Trump in the Presidency, even if it means temporarily shredding the Constitution in the process". They would probably get votes for honesty.
I think it is a gamble to not press the subpoena issue and it isn't how I would have run this. I would have given subpoena's, thrown their asses in jail when they didn't show up (which Trump would pardon them for, further weakening his position), and then held the vote anyway using those actions as further evidence of the GOP's guilt. I would probably move to impeach members not showing up for a subpoena if they still held an office as well. Not that that would go anywhere, but I would at least show the country I was doing everything I could to bring these people to justice. At the very least, I would show why I needed their help in 2020.