The reason the spellings are different is that reinforce does not mean "to enforce again". The word enforce is defined by the Macmillan dictionary and others as
to make sure that a law or rule is obeyed by people
to make sure that something happens or is done
Reinforce, however is defined as
to make an idea, belief or feeling stronger
to make a situation, process or type of behaviour stronger and more likely to continue
to make a building, structure or object stronger
to make a group of soldiers, police etc stronger by adding more people or equipment to it.
In short enforce means to ensure that a law or rule is obeyed or complied with, while reinforce means to make something stronger in some way.
There is no word "to inforce" meaning to make strong initially, if a building is made strong initially, say by the use of reinforced concrete in its construction it is said to be "strongly built" and, if it is later strengthened, it is then said to be reinforced.
Similarly there is no word reenforce meaning to enforce again, perhaps after a regulation has been allowed to lapse. If it was needed it might be hyphenated to give re-enfoce but the situation probably does not arise frequently enough for such a word to become universally understood.