For one, the tags aren't necessary -- you know what belongs at any point, and for OPTIONAL/DEFAULT elements all you need to know is whether they are present or

For one, the tags aren't necessary -- you know what belongs at any point, and for OPTIONAL/DEFAULT elements all you need to know is whether they are present or absent (that's one bit, or a byte if you're not aiming to be more space efficient). Definite-lengths greatly complicate encoding. The easiest way to encode DER is "backwards", starting the end of a SEQUENCE and working your way backwards. That's a bit of a pain. Definite-lengths add lots of mostly-unnecessary redundancy. For constrained-range integers, the best thing to do is to encode them as whatever number of octets are needed to encode the constrained range. For structures (SEQUENCE) and arrays (SEQUENCE/SET OF) indefinite length encoding is much easier to encode. Decoding is easy because you just decode the fields -- a terminator shouldn't be needed (though I know BER uses terminators, but that's a design flaw in the BER family of encoding rules, not a complete necessity. Definite-length encodings or terminator encodings are mostly only needed for strings and extensibility holes (OCTET STRINGs, extensibility markers), but they can be the exception rather than the rule, and thus simplify things.