Why did impeachment continue to be an issue for presidents into the 21st century?

Impeachment can be defined as to accuse a public officer of misconduct. The word is a combination of Latin words which means to accuse. As per the US constitution, only the house of representatives can impeach an official and senate can impeach the officials of the federal level and two-third majority is required to convict someone. If a president is being impeached the Chief Justice must preside during the Senate trial. Two presidents who have been impeached are Andrew Jackson(1868) and Bill Clinton (1998), both of them were acquitted. Impeachment is an issue because it is a slow process, and the president either finish their terms or resign before he could be impeached.