How come wands are measured in inches?

On some of Pottermore's bonus material (chapter one) JK says that because the wizarding community does not follow muggle laws and customs, they had no reason to go metric. It is also noted that because they have magic, there is no reason witches and wizards shouldn't just perform difficult calculations using that method. (defeating the purpose of going metric in the first place). The process of going metric actually began around 1965, and was an ongoing project until it became practically finalized in about 1980, (although some details are still being resolved today).

The books were, indeed, written in Great Britain. However, the series was started in the late 1990s. Thus, the metric system had not yet become the international standard. When it did, the measurements in the books were not altered, as this would confuse readers. Also, America is the only country that does NOT use the metric system. Overall, a change in measurement would confuse many readers.
It could also be that the Wizarding world is so much further behind the Muggle world that wizards never accepted the metric system.

Many people in the UK (and it is the rule in the US !) still use the Imperial system (= non metric). No wonder then.

Besides, when you want to infuse a particular atmosphere in a book, you have to use the codes of the people you describe, whether the way they dress or talk, what they drink and eat, etc. The HP series does not escape the rule. Why not using an antiquado, à moda antiga system then to describe people a bit antiquado, à moda antiga themselves ? (not to say downright out of place and time !)

A last explanation : the British wandmaker is Ollivander, whose family has made wands since... a long way back, before Christ but I can't remember the exact date. For several centuries then. I'm not surprised they have kept the same standards.

It reminds me of another field where people have kept similar standards : yachting - we still count in yards, miles, etc. The same can be said of aeronautics, they use feet to measure altitude - not meters or kilometers.