ASN.1 has too much stuff. The moment you write "I made ASN.1 decoder/encoder", someone will throw TeletexString or BMPString at it. Or inheritance, as morshu9001 sad. So at this point:

- You can support all those features, and your ASM.1 library will be horribly bloated and over-engineered.

- You can support your favorite subset, but then you cannot say it's ASN.1 anymore. It will be "ASN.brabel", which only has one implementation (yours). And who wants that?

(unless you are Google and have immense developer influence... But in this case, why not design things from scratch, since we are making all-new protocol anyway?)