Top 5. 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.

原因五：（罗马式）拼音方案太多且都让人精疲力尽。
也许听起来有些刺耳，但汉语的拼音方案确实太多，而大多数的拼音方案不是什么文字委员会提倡的，就是语言学家提出来的，更糟糕的是由那些语言学家组成的文字委员会提出来的。当然，要发明一种罗马式拼音方案确是一项技巧性很强的高难度工程，尽管有些方案比其他方案好，但任何方案都含有些无法凭直觉读的拼写。