Prevalence of diagnosed dental fluorosis is increasing (though in spite of the loaded phrasing in the question it's generally not an actual health risk) and there are probably at least three reasons:

1. Misdiagnosis
2. Improved diagnosis
3. Increased fluoride exposure.

The fluorosis that's increasing is, to a large extent, mild and very mild fluorosis (Current Guidance for Fluoride Intake: Is It Appropriate?; Dental Fluorosis: Chemistry and Biology). That means that the symptoms are not as dramatic as the classic severe fluorosis, and because there are many other disorders that look like mild fluorosis, there's a fair bit of misdiagnosis:

>Thus, misdiagnosis of non-fluoride-induced enamel defects may occur frequently. Reports of unexpectedly high population prevalence and individual cases of fluorosis, where such diagnoses are irreconcilable with the identified fluoride history, highlight the necessity for a more precise definition and diagnosis of dental fluorosis.

--Dental Fluorosis: the Risk of Misdiagnosis—a Review

Because there's increasing awareness of fluorosis it's also more likely to be correctly diagnosed today than it was 20 or 40 years ago, meaning that even if there was no increased prevalence there would still be increased diagnosis.

Finally, even taking away those factors, there is a genuine increase in mild fluorosis, and the reason is probably just what you'd think - Because adding fluoride is so effective and so beneficial, fluoride has been increasingly added to more and more products. When water fluoridation was introduced, the assumption was that people wouldn't be getting fluoride from other sources. Today, since that's obviously no longer true, there's movement toward revising water fluoridation guidelines, but since the current levels are clearly still safe as well as effective, the moves to reducing fluoride levels are cautious so that the baby isn't thrown out with the bathwater.

>In the present review, we discuss the appropriateness of the current guidance for fluoride intake, in light of the windows of susceptibility to caries and fluorosis, the modern trends of fluoride intake from multiple sources, individual variations in fluoride metabolism, and recent epidemiological data. ... An "optimal" range of fluoride intake is, however, desirable at the population level to guide programs of community fluoridation, but further research is necessary to provide additional support for future decisions on guidance in this area.

--Review of Fluoride Intake and Appropriateness of Current Guidelines