For very big numbers (that could appear in these fields), generating and parsing a base 10 decimal representation is way more cumbersome than using their binary representation.

The DER encoding used in the TLS certificates uses the big endian binary format. OpenSSL API wants the big endian binary too.

The format used by this protocol is a simple one.

It's almost exactly the format that is needed to use these numbers, except JSON can't store binary data directly. Converting binary to base 64 is a simple operation (just bit twiddling, no division), and it's easier than converting arbitrarily large numbers between base 2 and base 10. The 17-bit value happens to be an easy one, but other values may need thousands of bits.

It would be silly for the sender and recipient to need to use a BigNum library when the sender has the bytes and the recipient wants the bytes, and neither has use for a decimal number.