1. Why do we call a.e.i.o.u as vowels?
2. Why do we call the others as consonants?
These are not vowel sounds. They are vowel letters. Traditionally, there are five vowel letters in English and twenty-one consonant letters. The vowel letters are
A E I O U
and the consonant letters are
B C D F G H J K L M N P Q R S T V W X Y Z
As you can see Y is among consonant letters, even though it represents a vowel sound in the word "happy". The trouble with letters and sounds is they are not in a
bijective correspondence. One letter can represent many sounds and one sound can be represented by many letters. But as a rule of thumb, vowel letters are used to represent vowel sounds and consonant letters represent consonant sounds. There are very many vowel sounds in English, much more than just five. You can find something about English vowels
here. As you can see, they are definitely not A, E, I, O and U. There is an important difference between sounds and letters.
For words without vowel letters, there is "Mrs." for example. But it does have vowel sounds, two in fact.
There are words without vowel sounds in my native language, that is "w" and "z", which mean
in and
from respectively. In most cases, it wouldn't be possible to notice that they are separate words in normal speech. When I say "w kraju" (
in the country), I pronounce it ['fkraju], as if the two words were one word. The same goes for "z kraju" (
from the country), which is ['skraju].