Because there are too many romanization methods and they all suck.因为字母化方案太多了，而且全都不给力。

Well, 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.11 And if you're serious about a career in Chinese, you'll have to grapple with at least four or five of them, not including the bopomofu phonetic symbols used in Taiwan. There are probably a dozen or more romanization schemes out there somewhere, most of them mercifully obscure and rightfully ignored. There is a standing joke among sinologists that one of the first signs of senility in a China scholar is the compulsion to come up with a new romanization method.
嘛，这么说可能有点过分。不过真的，把中文用字母表达的方案很多，而绝大多数都是由某个委员会或是某些语言学家弄出来的。有时候还更糟，是个语言学家组成的委员会…… 当然啦，设计一种字母化方案非常不易，有些方案比较好，但所有的方案都需要很多与直觉抵触的拼写。而如果你真想发展中文方面的职业道路，那你至少得会其中四五种，还不包括台湾用的那些鬼画符。总共恐怕有超过一打的字母化方案，大部分都是晦涩难懂而理所应当地被大家忽略了。长久以来在汉学家之间有个笑话：一个汉学学者老年痴呆的标志，就是他感到发明一种新的字母化方案的迫切性。