According to Chhandogya Upanishad,
अस्य लोकस्य का गतिरित्याकाश इति होवाच
सर्वाणि ह वा इमानि भूतान्याकाशादेव समुत्पद्यन्त
आकाशं प्रत्यस्तं यन्त्याकाशो ह्येवैभ्यो ज्यायानकाशः
परायणम् ॥ १.९.१॥
I-ix-1: (Salavatya) 'What is the essence of this world?' 'Akasa' said (Pravahana); 'All these beings arise from Akasa alone and are finally dissolved into Akasa; because Akasa alone is greater than all these and Akasa is the support at all times.'
Adi Shankaracharya Bhashya on this verse:
BHAYA- " What is the essence of this Region?"- " Akasha' '-said Pravlihana. " Akasha" here stands for the Supreme Self; as the Vedic Text says-' Akasha by name': as the producing of all things is the function of that Self, all
things becorne dissolved into that Self. The Text itself is going to declare later on that "I created Light. The Light becomes dissolved into the Supreme Deity. --ALL these things in this world,-animate as well as inanimate, mobile and immobile-are produced out of Akasha,-in a regulated order of Light, Food and so forth, in accordance with the capacity of things ;-they become dissoloed into Akasha, at the time of Universal Dissolution,-in the reverse order:-because Akasha indeed is greater than these-superior, larger, than all things; and for that reason, it is the' param '-'ultimate'-'ayanam. -' substratum', basis, support, at all three points of time.-1)
Thus, Akasha is the greater/superior and supporter of all other elements and hence it is considered a basic element.
Also note that Akasha doesn't only refer to/as-an element, it is the narrator/signifier of Brahman according to Brahma Sutra 1.1.22:
आकाशस्तल्लिङ्गात् ॥ १.१.२२॥
22. Space (Akasa) is Brahman, for Brahman's indicatory mark is in evidence.