how come in the middle ages, wealth equaled power?

http://en.wikipedia.org/wiki/Middle_Ages

Wealth meant a man could have land, substantial housing, armor, weapons, horses, food for his horses, people who could care for his armor and weapons, etc.

These kinds of things (metal things, horses, food, etc.) were expensive. Most people didn't have the means to own them. Those who did have these things could attract others to band with them and form armies, small and eventually large.

Remember, in the Middle Ages, there was just about no middle class -- there were the wealthy and the poor and not many in between.

Other teachers may have more information to add.

=)