Why is it hard for the animals to use tools?
The animals found it difficult to use some tools, because they were animals. Even though the book is a fable of sorts, there are some realistic elements thrown in. One of them is the detail that the animals found it difficult to use tool that required them to stand on their hind legs. That said, Orwell also makes it a point to say that the pigs found a way around this, as they were extremely clever. Here are Orwell's words:
Sometimes the work was hard; the implements had been designed for human beings and not for animals, and it was a great drawback that no animal was able to use any tool that involved standing on his hind legs. But the pigs were so clever that they could think of a way round every difficulty.
Orwell also points out that the animals did have some some advantages, such as knowledge of the farm. For example, the horses knew the land of the farm better than Jones and his men. They also knew more about mowing and raking than Jones's men. So, even though the animals had some disadvantages, they managed.