why different micro controllers are used..? if u could make out a best of one kind,where many i/p o/p's and large memory space is available..? if u say that MCU's are chosen on its application basis, well order a MCU where the memory and i/p o/p's are configured by you.. so my outline of my question is that why we go for different micro controllers..? where it in-deed's the necessity to learn the programming languages individually for the micro controller we choose.!
Price, Availability, Size, Power Requirements, Different Architecture.
why different micro controllers are used
For the same reason they make more than one type of car. Or in fact more than one type of anything.
Just making one of everything because someone thinks it is the best is what lead to the downfall of communism.
Just making one of everything because someone thinks it is the best is what lead to the downfall of communism.
And the opportunity to create a need for variety is what grew capitalism and a consumption driven world! 
But the new board is called the Uno, it must be the only one to own.
Technology changes over time... but not all advances come from one source...
Usually, when one company leaps ahead, they set the standard that existing companies must/should exceed. This competition between companies continues to foster innovation.
As a result, a particular micro controller has a period where it is the latest "favorite". Give it a few years and technology advances will make that seem controller seem weak and undesirable.
I mean, not many of us would start a project with a PIC16F84 or a basic stamp now unless we had them lying around already. There was a time when nearly EVERYTHING was done with the PIC16F84.
Thank heaven for the innovators.
"The great thing about standards is that there are so many to choose from" 
One reason why Microchip is much more successful than Atmel is because they make far more devices. It's very easy to find a chip that meets the requirements for a particular application, at minimum cost.