Microsoft .NET Framework - a tool kit of different programs which are indispensible for running other applications. Most directly, it is a software framework some popular programs are built on and no chance they will work without it. Of course it’s a must have kit for every user.

Microsoft .NET Framework is aimed at applications based on .NET architecture. This architecture is Microsoft answer to Java and its main goal is to make different applications universal for various Windows OS.

I suppose certain people learned to program using .NET and they don't know any other way to program.

Some people may have started with and older version of Visual Basic and VB.NET seemed like a natural progression to them.

It is extremely easy and fast to develop for windows using .NET and you can very easily use the same languages for web development using ASP.NET. So for the most part it is just easy and fast.

Because by this we can develop software easily in very less time .

Because it works within the realm of the Microsoft web development world.