ZSTD is a great compression algorithm but an important PNG (v1.2) advantage is that implementations are available in almost all actively used operating systems and in most popular languages. The same cannot be said about ZSTD with very few implementations except https://github.com/facebook/zstd

I'm not even sure there is a good pure Java (no JNI) and Go (without Cgo) implementations for ZSTD. And it definitely would require more powerful hardware - some micro-controllers which can use PNG are too small for ZSTD.