1. Because the writing system is ridiculous
The beauty of the characters is indisputable, but these ideograms weren’t too practical for daily use. For one thing, it is simply unreasonably hard to learn enough characters to become functionally literate. Again, the Chinese writing system is harder to learn, in absolute terms, than an alphabetic writing system. It is absolutely true that Chinese is hard because of the huge number of characters one has to learn. Plus, reading comprehension is not simply a matter of knowing a lot of words; one has to get a feeling for how those words combine with other words in a multitude of different contexts. The problem of reading is often a touchy one for those in the China field and one has to conquer the inferiority complexes or fear of losing face. As if all this weren't bad enough, another ridiculous aspect of the Chinese writing system is that there are two (mercifully overlapping) sets of characters: the traditional characters and the simplified characters. Since they are routinely exposed to textbooks and materials from both Chinas, any foreign student of Chinese is more or less forced to become familiar with both sets.
2. Because the language doesn't have the common sense to use an alphabet.
The English is so easy because of the skills needed to master the writing system are 26 letters and they are written from left to right ,horizontally, across the page, with spaces to indicate word boundaries. In comparison, Chinese has nothing that corresponds to an alphabet, though there are recurring components that make up the characters. It can be said that the components of Chinese characters are arrayed in two dimensions, rather than in the neat one-dimensional rows of alphabetic writing.
3. Because the writing system just aren’t very phonetic.
Chinese is just not very phonetic, which means that often you just completely forget how to write a character, period. If there is no obvious semantic clue in the radical, and no helpful phonetic component somewhere in the character, you're just sunk. And you're sunk whether your native language is Chinese or not.
4. Because you can't cheat by using cognates
Cognates -- those helpful words that are just English words with a little foreign make-up. Recognizing these words as just English words in disguise is about as difficult as noticing that Superman is really Clark Kent without his glasses. That these quasi-English words are easier to learn than Chinese characters (which might as well be quasi-Martian) goes without saying. Imagine you are a diabetic in China, you'd be a goner for sure, unless you happen to have a dictionary with you, and even then you would probably pass out while frantically looking for the first character in the word for insulin.
5. Because even looking up a word in the dictionary is complicated
One of the most unreasonably difficult things about learning Chinese is that merely learning how to look up a word in the dictionary is about the equivalent of an entire semester of secretarial school. Chinese must also be one of the most dictionary-intensive languages on earth for there are various dictionaries. If you don't know the word in question, it comes another problem with looking up words in the dictionary that it takes quite a lot of knowledge of the language and often some genuine sleuth work to tell where word boundaries lie, thus it's often trial and error to look up a word.
6．Because there's classical Chinese (wenyanwen).
Whereas modern Mandarin is merely perversely hard, classical Chinese is deliberately impossible. Classical Chinese really consists of several centuries of esoteric anecdotes and in-jokes written in a kind of terse, miserly code for dissemination among a small, elite group of intellectually-inbred bookworms who already knew the whole literature backwards and forwards. An uninitiated westerner can no more be expected to understand such writing.
7. Because there are too many romanization methods and they all suck
Perhaps that's too harsh, but it is true that there are too many of them, and most of them were designed either by committee or by linguists, or -- even worse -- by a committee of linguists. It is, of course, a very tricky task to devise a romanization method; some are better than others, but all involve plenty of counterintuitive spellings.
8. Because tonal languages are weird
It’s one of the most common complaints about learning Chinese, and it's one of the aspects of the language that westerners are notoriously bad at. As non-native speakers, you must memorize along with the vowels and consonants. The real difficulty comes in when you start to really use Chinese to express yourself. Intonation and stress habits are incredibly ingrained and second-nature.
9. Because east is east and west is west, and the twain have only recently met
One of the main reasons Chinese is so difficult for Americans is that our two cultures have been isolated for so long. China has had extensive contact with the West in the last few decades, but there is still a vast sea of knowledge and ideas that is not shared by both cultures. When Americans and Chinese get together, there is often not just a language barrier, but an immense cultural barrier as well.