Because many letters of the English alphabet correspond to several different sounds and combinations of letters are no guarantee of set pronunciation.
Take the letter
A for example. It can be 'long' when we say
CAR (in the IPA this is shown like this:
a:) or it can be 'short' when we say HAT (in the IPA it's shown like this:
ae).
And famously the combination OUGH can be said in many different ways:
- R
OUGH 'ruff'
- THR
OUGH 'ooo'
- TH
OUGH -'o'
and so on.
If you're teaching pronunciation you should use the IPA as then there is no ambiguity about how a word is pronounced.
A basic IPA of English is here:
http://icalweb.com/wiki/index.php?title=IPA