The main reason that it is difficult to win such claims today is that governments no longer engage in overt acts of discrimination. Therefore, proving discrimination is very difficult.
In the past, governments were explicit in passing laws to disadvantage minorities. This made it easy to prove that they were violating the equal protection clause once that clause of the 14th Amendment was taken seriously. But today, no such laws exist anymore. If governments do, in fact, discriminate against minority groups, it is much harder to prove. For example, imagine that a certain school district has almost no Hispanic teachers even though the community is largely Hispanic. Is there any way to prove that race entered in to any single hiring decision? There really is not unless someone was irresponsible enough to mention race in writing.
Because discrimination is no longer overt, it is much harder to win equal protection claims today.