"I'll follow up tomorrow" feels harmless. You're not ignoring the lead — you're mid-job, tired, or waiting for a better moment to write something thoughtful. But the lead doesn't know that. All they know is they reached out and nothing came back.
By tomorrow, the problem that made them reach out is already handled by someone else, or it stopped feeling urgent enough to chase. Either way, the easy yes you had today is gone.
The fix isn't a perfect reply. It's a fast one. A ten-second "got your message, I'll call you by 3" keeps the door open while you finish what you're doing — a real answer can wait a few hours. Silence can't.