This observation made by Justice Brandeis  can often be seen on the web; it is not unknown:

We must make our choice. We may have democracy, or we may have wealth concentrated in the hands of a few, but we can't have both.
                    (As quoted by Raymond Lonergan in Mr. Justice Brandeis, Great American (1941), p. 42.)

Is it true that we can’t have both? Why can’t we have both wealth concentrated in the hands of a few, and democracy?

Fundamentally it is a question of power. Wealth gives power, and power can be infinitely creative in finding ways to impose itself no matter how others are affected: the power to treat people justly or not, the power to make decisions about the community without community input, the power to create jobs or not, the power to influence “elected” officials, the power to refuse dialogue and compromise, the power to impose one’s beliefs, the power to, in many other ways, make life difficult for others through economic manipulation and disrespect for needs of others.

Nevertheless, power can be used to help others, and sometimes is.

Uncontrolled power is the opposite of dialogue and compromise. It refuses to respect the wishes and needs of others, it does not accept limits to self-centeredness. People count on the values represented by democracy for protection.