Holdo doesn't tell Poe about her plan to send the smaller transports to Crait because he's reckless. As she and Leia both express, he's unable to get his head out of the cockpit. He's more interested in destroying the threat (at all costs) than necessarily doing what's best. Therefore, Holdo knows he'll disagree with her plan. She can carry it out only by forcibly knocking him unconscious (through Leia). Holdo is supposed to be a mirror of Leia, so Poe should've given her his total allegiance. The lesson for him to understand is one of meaningful sacrifice for worthwhile gains. It's something he eventually learns as he leads the Resistance to Rey while Luke Skywalker confronts Ren.