Short answer Because the “14 Sammūrcchanā” name broad, impure loci where spontaneously generated beings arise. “Blood (śoṇita)” is one of those broad categories. Menstrual blood is simply a specific case of blood, so it is already included and not listed separately.

A little more detail

• Sammūrcchanā means spontaneous (a‑sexual) formation of a body. Jain texts describe 3 modes of birth: garbha (uterine), sammūrcchanā (spontaneous), and upapāda (celestial/infernal). Menstrual flow pertains to garbha‑janma discussions, not to how sammūrcchanā beings arise.
• When texts list the “14 places/loci of Sammūrcchanā,” they name general impure substrates (like stool, urine, phlegm, pus, blood, decaying matter, etc.). The intent is to cover kinds (jāti), not every sub‑case. Hence menstrual blood needs no separate mention; it is covered under the general head “blood.”
• Both Digambara and Śvetāmbara traditions agree on the three birth modes and on the broad categorial nature of Sammūrcchanā loci. There is no separate doctrinal exception for menstrual blood.

If you wish to read a simple primer on the three kinds of birth and related basics, you can start here: