Why motorbike tires have maximum speed limit?

They are rated safe to that speed, If you plan on High Speeds, Get Tires rate for High Speed.
More Expensive and More Testing, better quality.

Please give actual reason

Generally it's an issue of heat build up in the tire. The faster you drive the hotter the tire can get, so high-speed tires need to have compounds that can better withstand the additional heat. Manufacturer's rate their tires for a mximum speed using a letter designation.

Check out this link, scroll down to "Speed".
Trailer Tire Facts - Discount Tire (http://www.discounttire.com/dtcs/infoTrailerTireFacts.dos)
I had Dunlop Qualifiers years ago on my 1100 Magna that had a "W" Rating, The only reason I would Go to 155 MPH was that all there was.
Is this a Test or are you buying tires?