The House failed to meet its responsibilities and threw the whole mess into the Senate’s lap.

Answer: Because the Democrats goal is not to obtain Boltons testimony. Rather, it is to tie the Senate in knots, extend the Senate trial as long as possible and inflict maximum political damage on the president and Senate Republicans. If Republicans let them get away with it, they will set a dangerous precedent.
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Unlike the House, the Senate cannot turn to other business in legislative session while an impeachment trial is underway without unanimous consent of all senators. If the Senate votes to call Bolton to testify, even if Bolton agrees to cooperate, the president would almost certainly seek an injunction to prevent him from doing so. That could tie the Senate in litigation that could last for many months.
Even if Bolton ultimately testified, during the trial the presidents defense team could object to his answering specific questions on the grounds that his answers would irreparably damage the presidency by divulging privileged information. The Senate might be forced to vote on every individual objection. And even if the Senate voted to allow Bolton to answer, and Bolton agreed to do so, the presidents lawyers could go to court to seek an injunction to prevent him from answering. In other words, there is almost no scenario in which the Senate is not dragged into court.
How long would the court battle take? During the Nixon impeachment inquiry, it took three months for the Supreme Court to rule on the presidents claim of privilege, but that was because the Supreme Court agreed to bypass the U.S. Circuit Court of Appeals for the District of Columbia. Most executive privilege cases take much longer.
Its impossible to predict how the courts would rule, but this much is certain: The presidents claims of privilege would not be dismissed quickly or lightly. Courts have accorded the president great deference on such claims when it comes to matters of national security, so they would very likely give similar weight to the presidents claims when it concerns conversations with his national security adviser. Moreover, in deciding whether to breach the presidents privilege, the courts would take into account the House managers testimony before the Senate in which they claimed they had proved their case without Boltons testimony.
But the bigger question is: Why should the Senate be stuck with this mess? There has never been a presidential impeachment trial in which the Senate was forced to resolve issues of privilege. That is the Houses job. If the House had subpoenaed Bolton months ago, the legal battle would be well underway. If representatives had done it even three weeks ago, when Bolton agreed to testify, we would be three weeks further along in the litigation fight.
Instead, the House failed to meet its responsibilities and threw the whole mess into the Senates lap. If senators agree to go along, they would set a precedent for future impeachment trials. Every three years of a new presidents term, the House could come up with a sloppy impeachment case based on second- and third-hand evidence, and then demand the Senate deal with it.
If you were a senator, what would you ask in the impeachment trial?
Not only have the House managers refused to do their job, they have also leveled outrageous accusations against senators if they refuse to do it for them. Rep. Jerrold Nadler (D-N.Y.) accused senators of being treacherous if they did not vote for witnesses an accusation that prompted Sen. Susan Collins (R-Maine) to send Chief Justice John G. Roberts Jr. a note complaining that Nadler had violated Senate rules. Rep. Adam B. Schiff (D-Calif.) drew audible gasps from senators when he declared that Trump had threatened to put their heads on a pike if they crossed him. If Democrats were really trying to convince moderate Republicans to call witnesses, they would not behave in such an ugly and partisan manner.
Why would the Senate agree to set such a precedent, especially when Boltons testimony will not change the outcome of the trial? A majority of senators agree that, as Alan Dershowitz said on the Senate floor Monday night, nothing in the Bolton revelations, even if true, would rise to the level of an abuse of power or an impeachable offense. Trump will be acquitted.
So, when the House managers demand that the Senate subpoena Bolton, the answer should be simple: Do it yourselves.
Read more:
Marc A. Thiessen: Democrats are hoping John Bolton will bring down Trump. Good luck with that.
Jennifer Rubin: No witness trades: Senate Republicans have lost leverage
Jennifer Rubin: Joni Ernst says the quiet part out loud
Kathryn Railsback:Senators confronting impeachment can learn from my fathers example in Watergate
Greg Sargent: The latest laughable Senate GOP spin further incriminates Trump