While going through the Rig Veda, I had found that the counting of numbers in Rig Veda was limited upto Ten thousand only (ayuta). Rig Vedic rishis did not mention about the higher numbers beyond Ten Thousand.
For example:
चित्र इद्राजा राजका इदन्यके यके सरस्वतीमनु । पर्जन्य इव ततनद्धि वृष्ट्या सहस्रमयुता ददत् ॥
citra id rājā rājakā id anyake yake sarasvatīm anu | parjanya iva tatanad dhi vṛṣṭyā sahasram ayutā dadat ||
Citra is the only king; the other petty little ones who (live) along the Sarasvatī are only kinglets— for like Parjanya with rain, he will thunder [/stretch forth] as he gives a thousand ten thousands.
——-
However, Yajur Veda, which came into existence at a much later date, contain large numbers like lakh (niyuta) or 0.1 Million and Crore (arbuda) or 10 millions.
Shukla Yajur Veda Samhita (Vajasaneyi) Chapter 17, mantra 2
इ॒मा मे॑ऽअग्न॒ऽइष्ट॑का धे॒नवः॑ स॒न्त्वेका॑ च॒ दश॑ च॒ दश॑ च श॒तं च॑ श॒तं च॑ स॒हस्रं॑ च स॒हस्रं॑ चा॒युतं॑ चा॒युतं॑ च नि॒युतं॑ च नि॒युतं॑ च प्र॒युतं॒ चार्बु॑दं च॒ न्य᳖र्बुदं च समु॒द्रश्च॒ मध्यं॒ चान्त॑श्च परा॒र्द्धश्चै॒ता मे॑ऽअग्न॒ऽइष्ट॑का धे॒नवः॑ सन्त्व॒मु॒त्रा॒मुष्मिँ॑ल्लो॒के ॥२ ॥
imā́ me agne íṣṭakā dhenávaḥ sántv ékā ca dáśa ca dáśa ca śatáṃ ca śatáṃ ca sahásraṃ ca sahásraṃ cāyútaṃ cāyútaṃ ca niyútaṃ ca niyútaṃ ca prayútaṃ cā́rbudaṃ ca nyàrbudaṃ samudráś ca mádhyaṃ cā́ntaś ca parā́rdhaś cāitā́ me agne íṣṭakā dhenávaḥ santu amútrā́muṣmiṃl lókē
These bricks of mine, O Agni, let them be milch-cows—one and ten, and ten and a hundred, and a hundred and a thousand, and a thousand and a ten thousand, and a ten thousand and a lakh, and a lakh and a ten lakhs, and ten lakhs and a Crore; Ten crores, a Hundred Crores, a One Thousand Crores, a ten thousand Crores, and the further half: these bricks of mine, O Agni, let them be milch-cows for me yonder, in that world.
——
Vedic Counting System (Ascending Order)
· Eka : One
· Dasha : Ten
· Shata : Hundred
· Sahasra : Thousand
· Ayuta : Ten Thousand
· Niyuta : Lakh
· Prayuta : 10 Lakh
· Arbuda : 1 Crore
· Nyarbuda : 10 crore
· Samudra : 100 Crore
· Madhya : 1,000 Crore
· Anta : 10,000 Crore
—-
Coming to the question part - What were the reasons for Rig Vedic people not going beyond Ten Thousand? - my understanding is as follows:
Rig Veda was composed at a time period, when the counting in Mathematics did not go beyond Ten Thousand, most probably due to the following reason:
The knowledge mathematics of Rig Vedic people might be upto that level only
If that was the case, then Rig Veda might have been composed at much, much earlier period to the composition of Yajur Veda.
