We know that Russia interfered in 2016, and intends to interfere in the future. By not taking the opportunity to make a public show of standing up to Putin, Trump risks reinforcing the very perception he has strained so vehemently to avert, one that lies at the heart of the federal investigation that has dogged his presidency: That he supported, or at least acquiesced to, Russian meddling in the 2016 campaign.
Worse, he will be signaling to Russia, and other hostile foreign powers, that he is unconcerned about foreign meddling in U.S. elections in the future, and will do nothing to stop it.
By not raising the issue with Putin, Trump will be saying, essentially: Go ahead. Hack away.