Here is Whitaker's statement on his scheduled testimony tomorrow. In a nutshell, he felt the Dems subpoena threat was in bad faith, so he's backing out of testifying unless they take it off the table.
https://www.washingtonpost.com/powerpost/house-panel-authorizes-subpoena-for-acting-attorney-general-whitaker-just-in-case/2019/02/07/1e5ec47a-2af2-11e9-b011-d8500644dc98_story.html?utm_term=.1756001fc87f