Small businesses lose customers quietly. Someone buys once, has a perfectly good experience, and then never thinks about you again, not because they were unhappy, but because nothing reminded them. Social media will not remind them: your posts reach a fraction of your followers and you do not control which fraction.
Email is the only channel where you decide who hears from you and when. It costs almost nothing per send, it works while you are doing the actual job, and the list is an asset you keep even if a platform changes its rules or disappears.
It also suits how small businesses really operate. You do not need a budget cycle or an agency retainer to send an email. You need thirty minutes and something honest to say.