If Trump were to offer some semblance of an apology, it would make it a lot easier for vulnerable Republicans to oppose impeachment without defending the underlying behavior. It would give them the added talking point, “Look, Trump has already apologized for his actions, but I just don’t think those actions rise to the level of removing him from office during an election year.”
It could also play into Trump’s own 2020 reelection campaign by demonstrating to persuadable voters that contrary to what people say, he does actually have the capacity to acknowledge mistakes and grow. And it’s not as if his loyal supporters will suddenly abandon him.
It’s undeniably true that Trump was unapologetic in 2016 and won the election. But it’s possible he won despite being unapologetic, rather than because of it. It’s also possible that the tactic worked in the past, but that different circumstances require different tactics.