The problem is that some of Trump’s defender have been claiming all of the testimony to date was ‘hear-say’. None of those people witnessed any “quid-pro-quo” from Trump directly. Trump has instructed anyone in the WH (Mulvaney/Pompeo) with such knowledge not to cooperate with the investigation (hence the obstruction of Congress charge). Bolton previously refused to cooperate, but since has stated that he would comply with a subpoena from the Senate. The House decided to go without the lengthy court battle it would take to get a ruling on the obstruction with the hope that they could get the Senate to call these witnesses. That’s why they’re asking for witnesses now.