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:
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