Nope, it's because the regex is wrong.

^  			// string's beginning
[aeiou]   		// a SINGLE vowel
$  			// end of string

Therefore this would match the vowels and only the vowels. Even a string like "aaa" would not match, because it's more than 1 char long.

The correct one is the one you used at the beginning because of the ".*" bit:

^			// start of string
[aeiou]			// a single vowel
.			// any characted...
*			// ...repeated any number of times
[aeiou]			// another vowel
$			// end of string