Because the meaning is different.
"Non era mia intenzione" means that I would want to not have did/said a specific thing that I did. Often it is a formula to beg pardon. e.g. If accidentally I offend you, I'll say you "scusa, non era mia intenzione" (sorry, it was not my intention - I don't know if this translation makes sense in English). Also can be that your actions generated unexpected (and not desired) things, and than you say that phrase. Therefore "Non era mia intenzione" simply means "non volevo" (I didn't want).
On the other hand, if I say "Non era la mia intenzione" (probably preceded from "questa"), that means that you didn't understand what was my purpose. "Non era la mia intenzione" means exactly "it wasn't my purpose".
I hope I helped...
Sorry, I still fail to see the distinction you're making. Saying exactly "it wasn't my purpose" in English would be the same as saying "it wasn't my intention." Both could be used as apologies.
I completely am missing your point in the second paragraph. Why would 'you' not understand 'my' purpose, when 'I'M' the one saying it wasn't my purpose?
The difference is that "Non era mia intenzione" is a formula, is equivalent to "non volevo" - "I didn't want". "Questa non era la mia intenzione" isn't a formula for apologies, but introduces an explanation about the purpose of your behaviour.
Excellent question - I don't know either. Sometimes the article can be dropped (eg, with family members: 'mio padre' instead of 'il mio padre'). I wonder if things that are part of you (eg, your intentions) are treated similarly.
Yes. I guess that like all language it comes down to usage rather than rules. Rules are useful so long as we don't get tripped up by the real-life exceptions to them.
It wasn't my intention is rejected. Does DL know about English contractions? Who is doing the translations? Are the translations done by a computer? If so please don't frustrate the people trying to learn.
This is absolutely absurd that DL reject an English contraction such as it wasn't my intention. Who is translated the sentences over there?