People who value the importance of everyone receiving a good legal defense - the same types of people who become public defenders.
The types of people who become public defenders are generally fresh out of a lower tier law school, granduating closer to the bottom of their class, who lack the connections to get a job elsewhere and lack the ambition to try to start their own firm. Almost nobody becomes a public defender when they have another option that actually pays what their degree is worth.
While I was considering becoming a lawyer, I briefly interned for a family friend who's a public defender. I found everyone at the public defender's office to be very competent and enthusiastic about their work, and for what it's worth said family friend graduated near the top of his class, is also a part-time professor at Boston College, and lives a reasonably comfortable life in an affluent suburb.
Sorry, I should have qualified my remarks
Some states have indeed entered the modern era and started paying public defenders decent wages; in Massachusetts, the the average starting salary for a Public Defender is ~$70k per year, and it's fairly common for more senior public defenders to have a six figure salary. Similar numbers for public defenders in federal courts throughout the country too, FWIW. Though contrast this with Georgia, for example, where the starting salary in state courts is ~$35k and on the county level can be as low as $20k per year (for a full time job after seven years of post-secondary education, yes).
