The reasons for which a language may have eight different words for ‘lion’ do not vary in essence from the reasons the Inuits have fifty words for ‘snow’, and ‘functionality’ at vast can answer the question.
Vis-à-vis the various cases PIE languages used for the inflection of nouns, it is worth emphasizing that there are still very much alive languages, which much like their precursors, haven’t dropped any of those markers throughout their existence: Russian (Именительный, Родительный, Дательный, Винительный and Предложный Падеж), Greek, Albanian (albeit, the latter had dropped its Vocative Case — one out of six Cases — some four or five decades ago: Rasa Emërore, Gjinore, Dhanore, Kallëzore and Rrjedhore). The various cases merely offer a particular suffix to the noun (singular masculine/feminine/neutral ; plural masculine/feminine/neutral).
From a linguistic perspective I could not possibly think of languages in terms of ‘complexity’. Each language has its own peculiarities, whether morphologically or syntactically speaking; it rises, evolves (in accordance with its existential circumstances), and in the best case scenario is preserved for a considerably long time. While Romance languages have little to no traces of noun inflection in the various cases, they make up for it with verb conjugations in Eight (!!) different modes (i.e., Italian: Modo Indicativo, Congiuntivo, Condizionale, Imperativo, Gerundio, Infinito Presente, Participio Presente, and Participio Passato) — not to mention the Tempi Semplici and Tempi Composti for each mode.
Semitic languages, such as Hebrew, while they share none of the above characteristics, they have other peculiarities. The verbs in Hebrew, although superficially seem to have only three simple tenses, are conjugated in both singular masculine/feminine and plural masculine/feminine according to the verb root structures called בנין Binyan — Structure: Binyan Po’el; Pi’el; HitPa’el; Hif’il; Pu’al.
In conclusion, it is safe to say that when a language is not established in written, there’s more flexibility and room for it to evolve (for instance, add/drop markers), as opposed to a well established, written language, and English is a perfect example of the latter. When Dante Alighieri wanted the lady of his interest to be able to read his poem, he chose to write it especially for her in the language of the peasants of his time, rather than in the official Latin. And by doing so, he gave birth and legitimized... Italian(!!). The poem was no other than La Divina Commedia (The Divine Comedy).
I hope that explains a bit the circumstances under which languages evolve.