Because that's pronounced "maintain-ance". Stress on the "ain".

That sounds like a regional dialect. I've never heard it pronounced that way in the northeast US at the very least.

I don't think anyone pronounces it that way, because what everyone is saying is the word that is spelt "maintenance".

If you were to actually pronounce the word that is spelt "maintainance", that is how it would be pronounced, with the stress on the "ain".

Bah. My bad. I confused the parties in the thread. I'm sorry for the noise.

'Twas not a problem!