Aristotle explains this himself in Book III of Politics. Aristotle's philosophy is a rich in complexity and detail, so this will be a synopsis. I highly recommend buying an annotated copy with lots of discussion. Preferably in a conventional style of English, not one of those terrible academic versions.
What is a citizen?
First, what is a citizen?
He who has the power to take part in the deliberative or judicial administration of any state is said by us to be a citizens of that state
(Book III, Part I, last sentence)
A citizen can administer the state. Throughout Book III, he seems to specifically be thinking of legislation and judicial decisions - although democracies of the time didn't have executives as we think of them now.
Aristotle doesn't just mean that a person must have the basic skills to serve these roles (listening, reading, etc.), but that a citizen should strive for a kind of excellence in these tasks. A good citizen should be knowledgeable in politics, should be able to analytically dissect arguments, and make good decisions for their community. Parts III - V of Book III outline this quite nicely. It should also be reminiscent of his entire idea of arete (excellence) ethics.
What about the masses?
Aristotle isn't against letting the masses rule because they are poor (not directly, anyway). The problem with working people is that they cannot dedicate enough of their own time or resources to suitably govern the state. Aristocrats don't have jobs - they can spend all their time and energy dealing with public problems. Workers, even those with a bit of leisure time, simply cannot devote enough of themselves to politics to ever have the knowledge or skills to be good rulers.
All of Book III deals with this issue. I would try to focus on Part IV and Part XI as they are especially relevant.
The majority of the people are mostly the poor, therefore they're not good rulers.
This is not Aristotle's argument. First, he isn't interested in being poor, his concern is about working people (those who have jobs to pay for their living expenses).
A more faithful reproduction might be something like, "The majority of people must spend most of their energy and time dealing with their own economic concerns. Being a good citizen requires considerable time and energy, which cannot be accomplished while still holding a job. Therefore, the majority cannot become good rulers."
He didn't mean people as whole by demos, he meant the poor, the plebs, the crowd.
Aristotle organizes society into two parts: the aristocracy and the many. He likens society to an individual, which is composed of soul and body. The aristocracy is the soul; the many are the body. The aristocracy can refine themselves into excellent rulers, while the many can enact their decisions. The many are not necessarily poor (men would have occupations and may have reasonable standards of living).