Why American & British English has different expressions?

Basically it's a geographical factor. Two nations developed differently in different areas of the world over centuries, so it's not surprising to develop a language quite differently. Actually this also happens to us. We HK Chinese, for example, talk about air-conditioners by using the term 泠氣 while those from the mainland refer them to 空調.

The difference can also be cultural. So we call buses 巴士 (influenced by the 'colonial ' English word BUS) while Taiwanese might just accept 公車.

One nation may be more conservative than another and so the people are less likely to respond to changes. Americans didn't feel uncomfortable, I guess, when they shortened or simplified certain words and usage (e.g. favour to favor; jewellery to jewelry; practise as v and practice as n, to practice as v & n too).

2007-01-05 11:16:06 補充：
disk - disc

2007-01-05 16:27:10 補充：
correction: air-conditioners are supposed to be 冷氣機 / 空調機

2007-01-05 16:32:50 補充：
You might be interested in the following link:http://esl.about.com/library/vocabulary/blbritam.htm

The English language has many variants around the world. In fact, even within Britain, there are regional accents and phrases - sometimes two towns that are merely 20 km away from each other can have completely different accents!

Don't forget even a small place like Hong Kong has its own variant of English. Words like kung fu, amah, tai-pan, shroff are HK's contributions to the rich and diverse world of the English language.

Please refer to the wikipedia article on American English for more info.