Because a value is an identifier only used under-the-hood. An identifier should comply to the Java convention of identifiers. Among them: do not start with a number.
Why do you need it? You can define captions for your enumerations. These are the only texts which are visible for your end-users.
I don't know why you can't use a number in a enumeration value name. However you can use numbers in the caption which is visible to the user. For example:
Name: e0 --> Caption: 0%
Name: e50 --> Caption: 50%
Name: e100 --> Caption: 100%