لِعَدَمِ الْعَلَمِيَّةِ فِي الْعُجْمَةِ
Translation: Because of the absence of proper-name status in the foreign language.
لِ: causal preposition; عَدَمِ: genitive noun and first term of iḍāfa; الْعَلَمِيَّةِ: genitive second term.
لِسُكُونِ الْأَوْسَطِ
Translation: Because the middle letter is sākin.
سُكُونِ: genitive first term; الْأَوْسَطِ: genitive second term.
Both causal phrases explain why the author counts the word as fully declinable.