We have to remember that the Vedic Sanskrit is different from that of Classical Sanskrit, which was commenced around 500 BCE.
The Post vedic literature like EPICs, all sutras, etc, were composed in Classical Sanskrit. So the way expressions used in Vedic Sanskrit /Rig Veda will be different, though we may get the similar meaning in the mantras of Rig Veda.
Rig Veda II.8.3,composed by the sage from Bhrigu family, eulogises AGNI as follows:
य उ शरिया दमेष्वा दोषोषसि परशस्यते | यस्य वरतं न मीयते ||
Who for his glory is extolled at eve and morning in our homes, Whose
statute is inviolate;
In this mantra, worshipping of AGNI, morning and evening (daily) was made mandatory.
We have to remember that the Savitr or AGNI, etc, are the epithets used in Rig Veda to eulogise the same Almighty.
Savitr/Gayatri mantra appears in Rig Veda at 3.62.10, which was later to the above Rig Vedic mantra II.8.3.
So worshipping of Savitr/AGNI, daily 2 times, might have been adopted for reciting Gayatri mantra also. As worshipping of AGNI daily was mandatory, reciting of Gayatri mantra was also made mandatory for 2 times.
We have to remember that even in the EPIC like Ramayana, recitation of SUPREME HYMN was mentioned for 2 times only, as Sandhya or twilight period appears only twice in a day. Performing Sandhya at Mid day might have been later day development.
संध्या काल मनाः श्यामा ध्रुवम् एष्यति जानकी | नदीम् च इमाम् शिव जलाम्
संध्या अर्थे वर वर्णिनी || ५-१४-४९ (Sundara Kanda)
"The ever youthful one with the best complexion, Seetha interested in
the rites of Sandhya time will definitely come to this river with the
auspicious water for Sandhya rite."