Mitas EO 7

I am extremely disappointed at the mileage I have got out of my rear tyre. I have religiously kept my tyre inflated to 1.5 bar. I have done just short of 4000 km's on the tyre with a 50% mix of tar and dirt. The tread depth in the middle is now 3.5mm and is still 9 mm on the side. I don't wheel spin unnecessarily or ride like a maniac. I was hoping to get at least 10 000 km out of them. I have been very happy with the performance so far. Is this normal? Any recommendations for a tyre with similar performance but longer lasting?