data-placeholder true | false - Can be set on option element of a non-native select
Note the option element in the description above. You're specifying the placeholder attribute on the select element instead of the option element that is supposed to be your placeholder. This attribute specifies which option element is supposed to be the placeholder, not that the select menu has a placeholder. How would JQM know which option is supposed to be the placeholder if it wasn't declared on a specific option element?