Because text literally has no tone. When you read it, you're left to determine what tone was used by using context of what's written around it(if anything).
Perfect example is in the movie 'The Martian' when they tell Matt Damon about the plan to get him off the planet using a tarp to cover the top of the capsule, while at the same time becoming 'the fastest man in space' or however they phrased it.
It shows the people trying to gauge his response when he replies with 'Are you f'ing kidding me?' and it shows them trying to decide whether he said it as if he were excitingly surprised or whether it was said with grudging incredulity. Then it shows the woman guessing that he meant it the first way, though she has no idea which he actually meant.
Perfectly shows this, which plays out every day, all across the world. And it still comes back to my first sentence. It's hard to convey tone of speech because there is no tone to convey. One must determine the tone using what's written around it.