As with most renewables (e.g., wind, solar, wave), I would imagine that just because production of energy outpaced its consumption on a given day, the same may not be true on any other given day.

It was not clear if energy capacity has recently increased but the article did state that last year on average it produced 39% from wind. That being the case, focusing on conservation seems like a good idea still...

Just a guess but I'm going to go with a common sense answer. Because usage can quickly outstrip supply.

Look at how Saudi domestic oil consumption has greatly increased, for example.

http://www.nytimes.com/2014/06/18/business/energy-environmen...

Update

Let me add another common sense answer. Because any excess electricity can be sold to other countries for a profit.