appendChild in for loop only adds 1 child
You are appending the same element over and over. You need to call document.createElement each time you wish to have a new element.
EDIT: If the element setup is really complicated and potentially includes children then you can also use Node.cloneNode
If you want to make one copy of the element, you will need to clone it. Use cloneNode()
So change
grid.tr.appendChild(grid.td);
to
grid.tr.appendChild(grid.td.cloneNode(true));