Because the judge or official who decided the case questioned the integrity of the people who were paid to collect the signatures and decided that the whole system of collecting signatures was probably crooked. I read that if he wants to run as an Indy, he needs to collect 3,000 signatures. LOL! As a write-in candidate, he'll have to rely on people knowing how to spell his name. He's also exploring the option of having a rubber stamp with his name on it available at the polls so that people can "write-in" his name without any trouble. I haven't heard whether they will allow him to do that yet.
If that isn't a ringing endorsement of school vouchers, what is?
In D.C., that means he's well-and-truly screwed.