* Flash is a scripting language. It's very hard for a fuzzer with no domain knowledge (like AFL-fuzz) to fuzz language-level constructs, since the vast majority of random inputs are trivially invalid.

* Flash is typically embedded in a larger program like a browser; this makes it much more expensive to test (browsers have long startup times, and are very slow when run under the kind of instrumentation needed to detect things like use after free bugs)

Unfortunately there's no silver bullet for finding vulnerabilities...