Because it would translate "This is the only way". You would express that there is only one possible way of solving the issue, while you were supposed to tell, that the given solution is only one of possible many.
This sentance implied that I have given you one option, but there are more to choose from. Does it imply the same thing in Polish? It is kind of... klunky. It took me a few reads to figure out why anyone would say this.