How do I download zip file in C#?

It's mainly because you use a StreamWriter : TextWriter to handle a binary Zip file. A StreamWriter expects text and will apply an Encoding. And even the simple ASCII Encoder might try to 'fix' what it thinks are invalid line-endings.

You can replace all your code with:

  using (var client = new WebClient())
  {
    client.DownloadFile("http://something",  @"D:\Downloads\1.zip");
  }

Note that for new code you should look at HttpClient instead of WebClient.
And then don't use using( ) { }

You could just use WebClient for a 2-liner:

using(WebClient wc = new WebClient())
{
   wc.DownloadFile(url, @"D:\Downloads\1.zip");
}