Then why would anyone follow them?
Consider it from the perspective of a Santa Muerte follower: You have little formal education in your faith (or anything else for that matter). Instead, your beliefs come from the traditions of your family, friends, and community. One day, a friend or co-worker approaches you and tells you all about about “Santa Muerte”, a new saint who grants incredible favors to her followers. You have a strong faith but don’t consider yourself to be a “good Catholic”; maybe you’re living with your girlfriend or dealing drugs. Unlike the other saints, Santa Muerte isn’t perfect either and she promises not to judge you.
So, you say a few prayers to Santa Muerte and you soon receive the favors that you requested (probably by concidence!) Maybe your mother recovers from a serious illness, your business has a profitable day, or you recieve a lucrative ransom for the businessman that you kidnapped. Now a believer in Santa Muerte, you lay an offering of flowers, tequila, or gold on her altar to repay her for the favor, and you become a new evanglist for her.
You still consider yourself a Catholic (because only the Catholics believe in saints) and Santa Muerte becomes a devotion, in the same way that other Catholics are devoted to the Divine Mercy or Saint Jude. Your priest is probably preaching against Santa Muerte, but you hardly ever go to Mass anyway so you never hear it.
This is how a folk religion gets started.
There is an interesting element of “bargining” in the Santa Muerte devotion which resembles how many American Catholics believe that a certain “miracle prayer” will guarantee results. You often see these on CAF, with a note that a certain prayer or novena has “never been known to fail”.