Not only are they small & hard to manipulate, but there's only 8 of them that are used to write all 26 letters of the alphabet!

eg, to get an 'A' you press the '2' key;
to get a 'B' you press the '2' key twice;
to get a 'C' you press the '2' key three times.

Some keys have 4 letters on them.

Typing 2 (or more) letters in succession that use the same key requires an extra keypress to "advance the cursor"

Thus, to type "you" into an SMS text message on a mobile phone takes nine key presses:
press '9' 4 times for 'y'
press '6' 3 times for 'o'
press '8' 2 times for 'u'
Hence you can see why people would want to abbreviate this to 'u' (only 2 key presses) when typing on a mobile phone.

But writing 'u' instead of "you" when you have a full PC keyboard is pure laziness!

(modern phones have predictive SMS text entry and other cunning tricks to alleviate this problem anyway).