Most numerology numbers use an entire birth name. The soul urge number is the exception that only wants part of it: every vowel, and nothing else, converted through the same Pythagorean letter-to-number chart used elsewhere in the system.
The logic the tradition gives for this split is more poetic than technical — consonants are treated as the "shape" of a name, the part other people see and pronounce, while vowels are treated as its "breath," carrying something closer to the person underneath. Add up the vowel values, reduce to a single digit, and that's the soul urge number.
Whether that consonant/vowel divide means anything beyond being a memorable way to organize a calculation is not something numerology or anyone else has demonstrated — but it's a clean enough idea that it has stuck around for a long time.