How did Hittite and Luwian preserve ancient noun forms with alternating r and n suffixes?

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Map of ancient Anatolia and northern Syria showing the geographic distribution of Anatolian languages, including Hittite, Hattian, Lydian, Phrygian, Palaic, Carian, Lycian, Pisidian, Hurrian, and Urartean. Major historical sites such as Hattusa, Kanesh, and Ugarit are marked, with the Black Sea and Mediterranean Sea bordering the region. — Anatolian languages Distribution of the Anatolian languages.

Hittite and Luwian preserve an archaic type of noun known as a heteroclitic noun, which features alternating -r and -n suffixes depending on the grammatical case.12

In these languages, the stem of the noun changes form to mark its function within a sentence. Typically, the -r suffix appears in the nominative and accusative cases, while the -n suffix appears in other cases. Examples of this ancient pattern include:2

Hittite: wātar (nominative/accusative) and witenas (genitive), meaning "water".12
Luwian: dū-r (nominative/accusative) and dū-n-ati (instrumental), meaning "urine".1

This feature is considered a striking archaism within the Anatolian languages, reflecting an irregular inflectional system that deviates from more common patterns.132