Speaking as one Yorkshireman to another…
The thing is, that in 'proper Yorkshire-ese' when spoken it would likely come out closer to 'Why want [or wənt] you…' with little differentiation between whether the word being substituted was originally wasn't or weren't.
Similar to the 'would of, could of' problem, it's heard so frequently in its abbreviated form that people become unaware of what the substituted word should actually be.
The 's' disappears entirely*, making the differentiation harder, unless you are to write it - when it once again becomes an important distinction.
I don't see how this can be explained as 'dialect', unless, as alphabet points out, we're talking that small area of 'heaviest' Yorkshire - between Sheffield, Barnsley & Doncaster - though I worked many years inside that accent zone, one of my nieces comes from there too; and it's not something I've come across in anything other than very casual speech. I don't know of anyone who would write it that way.
*The same happens with the 'd' in wouldn't & couldn't which come out as 'wunt' - you can guess the other - and sounds to southerners like you're swearing.
This, though, creates no confusion when written.
Just to make it fully clear, it's…
I was
He was
You were
We were
They were.