Why "monoxide" but not "diodine"?
Both &quot;monooxide&quot; and &quot;monoxide&quot; are used in the literature, yet &quot;monoxide&quot; is being  used more often (Google Books Ngram Viewer).
Although this is an accepted elision, it is not the preferred one, and must not set a precedent for other cases when multiplicative prefix ends with the same vowel as the root word begins with, such as &quot;diiodine&quot;.
According to the current version of Nomenclature of Inorganic Chemistry, IUPAC Recommendations 2005 [1, p. 31]:

IR-2.7 ELISIONS
In general, in compositional and additive nomenclature no elisions are made when using
multiplicative preﬁxes.
Example:

tetraaqua (not tetraqua)
monooxygen (not monoxygen)
tetraarsenic hexaoxide

However, monoxide, rather than monooxide, is an allowed exception through general use.

Further, from section IR-5.2 Stoichiometric names of elements and binary compounds [1, p. 69]:

The multiplicative preﬁxes precede the names they multiply, and are joined directly to them without spaces or hyphens. The ﬁnal vowels of multiplicative prefixes should not be elided (although ‘monoxide’, rather than ‘monooxide’, is an allowed exception because of general usage).
[…]
Examples:
[…]

$\ce{NO}$ nitrogen oxide, or nitrogen monooxide, or nitrogen monoxide


References

IUPAC. Nomenclature of Inorganic Chemistry, IUPAC Recommendations 2005 (the “Red Book”), 1st ed.; Connelly, N. G., Damhus, T., Hartshorn, R. M., Hutton, A. T., Eds.; RSC Publishing: Cambridge, UK, 2005. ISBN 978-0-85404-438-2. IUPAC website