it's not a horrible airport - it's small and so easy to navigate; getting in and out of SAW takes half the time it does at IST
maybe more people have connecting flights at IST or are travelling here for business/tourism purposes and need to be on the european side. that's my guess
ETA: the first sun express flight out of SAW is in less than 24hours; looks like they fly to antalya and many other turkish or european cities
http://www.sunexpress.com.tr/