Printing 2D array is more organized than it's supposed to be

I am attempting to randomly place the letter "x" into slots in a 2D array as an initializer for a Conway Game of Life.

        string[,] initialArray = new string[rowMax, colMax];

        for(int row = 0; row < rowMax; row++)
        {
            for(int col = 0; col < colMax; col++)
            {
                if(randomNumber() < 7)
                {
                    initialArray[row, col] = " ";
                }
                else
                {
                    initialArray[row, col] = "x";
                }
                tbGrid.Text += initialArray[row, col];
            }
            tbGrid.Text += "\r\n";
        }

What I want to appear is a somewhat random placement such as :

       x     x           x            xx        xx      x
                    x            x           x         

etc.
However, what I'm ending up with is more grouped together such as:

        xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
        xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx


        xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
        xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Just for example.

Any idea why that might be? At first I thought it was because I had the tbGrid.Text statement inside the IF statement. Taking that out helped, but it still didn't perform as desired.

The random number generator is set as follows:

    public static int randomNumber()
    {
        Random randomNum = new Random();
        int random = randomNum.Next(1, 10);
        return random;      
    }

If that's helpful. Ideas? Suggestions?