Because the city had two building booms with sixty years between them, and both left their stock standing. Delray Beach had its golden age of architecture during the Great Depression, when it ranked fiftieth in the country by population and tenth by building permits. That produced the pre-war housing that five historic districts now protect, all of it between US Route 1 and the Intracoastal Waterway.
Then downtown revitalization began in the 1990s and substantial mixed-use development has continued since 2003. So a block can hold a house from the 1930s and a building from last decade, with completely different plumbing behind each.
For a homeowner that means the useful question is never what Delray Beach is like. It is what your particular building is, and whether it sits inside a district where the permit is not the only approval that matters.