Because the demand for HiDPI desktop displays isn't there yet. You would buy one, and so would I, but overall they wouldn't sell much, compared to the standard 1080p panels. We have to wait another year until Apple introduces Retina iMacs, tells everybody how awesome they are and why people absolutely need Retina displays.
Then suddenly there will be steep increase in the demand of HiDPI panels, and a few month later every manufaturer starts offering them. If I learned anything the last few years, it's that's how it works.