I was looking for some off road mtb 28" tires and couldn't find anything suitable . There were only some tires for road or hybrid bikes .Then I started looking for 28er mtbs and could't find anything also . So what is the reason that they don't produce any 28er mtbs ???
I assume you are looking for a cube CLS LTD PRO based on other questions you asked. If so, that bike likly came with Schwalbe Smart Sam, Active 622x44 tires.
Tire sizing terminology is a bit weird. Despite the same sized rims, mountain bike companies and road bike companies use different terms to describe tires that fit the same rim sizes. It is even more confusing due to mixing of metric and imperial measurement systems.
If you're looking for the same or smaller tires you'll probably want to look for 622x44 size. The first number is the rim size (622mm) and the second number is the tire width (44mm).
If you wanted larger tires, they would probably be described differently since they would likely be mountain bike tires, they would be described as 29x2.2 (which effectively means 622mm rim with 2.2 inch tires). The 29 is to describe the effective outer diameter of the tire, not the rim size, you may even see it described as 622x56 instead of 29x2.2, but it's hard to say.
Sheldon brown has a good page that goes in to detail about various tire and rim sizes and may be better at describing the oddities of tire and wheel size descriptions, it's a good read. http://sheldonbrown.com/tire-sizing.html