The Republicans have a slim majority in the U.S. House of Representatives. Last year, eight Republican Congressmen filed a motion to vacate the Speakership,which succeeded, because the former (Republican) Speaker Kevin McCarthy had lost his majority. Then the Republican-led Congress went through several candidates before settling on Mike Johnson.
Even though they had too few votes to elect their own Hakeem Jeffries, the Democrats (or a few of them) could have voted to save McCarthy. In failing to do so, did they have a plan (perhaps an unsuccessful one) to get a Speaker more to their liking? Did they make a political calculation to let the Republicans fight it out among themselves? Or did things get out of hand so that the final result was "random?"
Put another way, is Mike Johnson more acceptable to the Democrats than Kevin McCarthy, and if so, how? Was this because of policy (e.g. "moderation" or lack thereof) or other grounds? Or was the final result a matter of "potluck?" Would it have made sense for the Democrats to have supported Tom Emmers or one of the numerous other candidates, or were they call comparably "bad" from the Democrats' point of view.
Note: The answers have alluded to "style" rather than "policy" reasons for the Democrats letting McCarthy fall.