Democracy might advocate that every vote is equal, the but the reality is that every vote is not equal. People's opinions are shaped by their differing environments; how educated they are, whether or not they are on the right side of the law, etc. If, for example, a person is uneducated or does not follow the news, then they would be very hard-pressed to make an objective, rational, and informed decision, because they simply do not have all the facts.
Should their vote count just as much as a person who has been well-educated on current issues and on how voting works? Wouldn't it just skew the outcome of an election and not be a true representation of what the population really wants? Couldn't it even allow for uneducated voters to be taken advantage of and manipulated into voting for someone who doesn't truly represent what they want?
And in regards to equal education and opportunities, these simply do not exist. Schools are notoriously unequally funded, because they rely on taxpayers' money--and poor neighborhoods generate less revenue to support their schools, while rich neighborhoods have most everything they could ever need.
There is no opportunity for equal mobility, because the under-funded education system in poor neighborhoods fails to deliver. Kids from poor neighborhoods start out at a disadvantage to achieving their goals, to starting careers, and to moving up and out of poverty. If democracy actually worked, this gross inequality would never be allowed. And by allowing inequality to persist in the quality of education the population receives, we are guaranteeing that votes will remain unequal, and that election results will continue to be skewed and voters abused.