Tchaikovsky, Prokofiev, Adolphe Adam and many others of their time, composed music specifically for ballet. When a new ballet was commissioned, there was someone writing a score for it, then someone making a choreography.
More recently, new classical ballets do appear, but they are choreographed to music that has not originally been written for the purpose. Examples of such ballets are L'histoire de Manon and Mayerling.
Since, clearly, there is demand for new classical ballet, why are new scores for ballet not being written, or at least not written nearly as often as new ballets are made? E.g., Alice's Adventures in Wonderland is the first full-length score for the Royal Ballet in 20 years (source). Why 20 years with no new scores, and then a ballet for children, as if afraid to write for adults?