Hello,
I'm beginning in Objective-C development and as i can see in the development blogs(about Objective-C) all they use Mac and some use Linux, but why we don't have many Objective-C developers on Windows, since Objective-C is a very good and easy language?
AFAIK, Objective-C is only used by Apple, thus the high use on Macs. Objective-C is a great language, and it might start seeing exposure on other platforms, though. If you want to code for Mac or for iPhone, pretty much the only options are Objective-C.
A big part of the answer is tools. As far as I know, there's no compiler that runs on Windows, no framework for Objective C on windows, nothing. It's not a great place to start from.
If you want a C-derived language for Windows, go with C#. Visual Studio Express editions are free, if you want to give it a try.
Objective-C alone is not so great. What makes it wonderful is mainly Cocoa, which is available only on Mac.
Frameworks.
Apple development is mostly done using the Cocoa Objective-C frameworks. Without similar frameworks on Windows, there isn't much reason to use the language.
Obj-C is mainly used on Mac because Apple champions it heavily, uses it for their API's and provides a lot of tool support for it.
It's pretty much the same reason why so much C# development is focused on Windows. Microsoft uses it for many of their products, they provide probably the best tools in the industry for working with it.
For the language to catch on on Windows, the Windows platform has to support it, and do so well. And I don't think that's going to happen.
The reason is, as many have stated, that the frameworks and tools are primarily Apple's. However, there are alternatives. Two I know of:
GNUstep is a better choice for working directly on Windows; Cocotron is better for building on Mac and cross-compiling to Windows. Good luck!