I don't think the vacancies are deliberate. Trump has been unusually slow in nominating people in general
The Senate has confirmed 26 of Trump’s picks for his Cabinet and other top posts. But for 530 other vacant senior-level jobs requiring Senate confirmation, the president has advanced just 37 nominees, according to data tracked by The Washington Post and the nonpartisan Partnership for Public Service’s Center for Presidential Transition. These posts include the deputy secretaries and undersecretaries, chief financial officers, ambassadors, general counsels, and heads of smaller agencies who run the government day-to-day.
That’s less than half the nominees President Barack Obama had sent to the Senate by this point in his first term.
But the nomination process has also been slowed by the unusual degree of scrutiny the White House is giving job candidates. Prospective nominees for senior posts and even some of the more junior ones must win approval from competing camps inside the White House, according to close Trump associates and Republican lobbyists.
As to why there's two vacancies, Democrats were unable to get their nominee confirmed before Obama left office and Trump withdrew her from consideration
Rosenworcel, a Democrat, left the FCC at the end of last year after her tenure expired. President Barack Obama renominated her just weeks before he left office. The apparent expectation was that once Trump took office, he would pair her nomination with a Republican choice and they would jointly go through the confirmation process.
But Trump’s decision to pull her nomination has led to speculation that he would put forward another Republican and perhaps an independent or other Democrat more favorable to administration policy. In the past, the White House has deferred to Senate leadership in the selection of nominees from the opposing party. Democrats have already been vowing to push back if the administration tries to buck that tradition.