The original design of Unicode allowed the inclusion of some 65,534 characters, with some six thousand of those "private use characters" which basically anybody can use any way they like. When it became clear that this simply wasn't enough to accomodate all the large, rare character sets such as Egyptian hieroglyphics, and uncommon Han ideographs, an extension mechanism (surrogates) was added to the standard which allow the addition of another million characters.
The problem was, a vicious circle quickly arose. Nobody started to implement surrogates because there were no characters encoded using them, and nobody wanted their characters to be encoded using surrogates because nobody was implementing them.
To break the vicious circle, we needed a writing system which was at once real and at the same time so incredibly rare and/or dead that nobody would object to its being encoded with surrogates. I volunteered the Deseret Alphabet as such as script, and it was quickly accepted. Since then, Shavian, the Cypriot syllabary, Linear B, Gothic, and Etruscan have all been formally approved for addition to Unicode using surrogates and dozens of other scripts have been earmarked for encoding that way. The Deseret Alphabet has also been approved by WG2, the ISO standards body which works with Unicode on the continued development of Unicode's ISO counterpart, ISO/IEC 10646.
The original proposal: