Why missing characters and empty code points?

[admin at genome.arizona.edu]

For example, why is MATHEMATICAL SCRIPT SMALL O missing and the assumed
code point it would have, 1D4C4, is empty?  Yet 1D4C3 (MATHEMATICAL
SCRIPT SMALL N) and 1D4C5 (MATHEMATICAL SCRIPT SMALL P) are defined. 
Makes no sense.  Thanks