Why are telephone wires left slack in the summer?

Bofore the question can be answered, it needs to be properly rephrased.

The question contains a grammar error that is all too frequent these days... non-agreement with the subject in number (singular or plural). We need to be more careful of such simple grammar slip-ups. Poor grammar betrays a lack of education and an imprecision of thought.

Correction: Why are telephone wires left slack?

This error is creeping into speech today...from even the news anchors... through the contraction of "is" following the Word "there", e.g. "there's too many jobs being lost...".

Correction: There are too many jobs being lost...