Because it's a chance. Should Roshi get another standard attack if he doesn't cause repulse with the first? He had a chance to dodge the first attack, and he didn't that's what a chance is, you may or may not be able to. If he didn't dodge it, then his passive CHANCE wasn't in his favor. Units that can cause certain damage and a chase starter are utterly pointless. Why does it matter if it causes knockdown? He didn't dodge the first, so he will just dodge the incoming chase.

In short, a chance should be seen as a chance, not a pity gift. IF they have a chance to do something, it doesn't mean they should get a free pass for another one if they don't.