The translations are not so different from one another as they might, at first, otherwise seem.
The first two main differences are found in the phrase "ten thousands" and "myriads" and "saints" and "holy ones".
The key here is knowing that the word myriad originally meant ten thousand, and then later, any sufficiently large number.
And the word saints means holy ones.
https://www.merriam-webster.com/dictionary/saint (from the sense of something be sacred)
The question then merely revolves around the final phrases "fiery law", and etc.
As you might see in the footnote [b] for the ESV, the translators wrote the following:
The meaning of the Hebrew word is uncertain
The NIV has a similar footnote [b]:
The meaning of the Hebrew for this phrase is uncertain.
In Professor Richard Elliot Friedman's Commentary on the Torah: With a New English Translation and the Hebrew Text, for Deuteronomy 33:2, he has the following note:
33:2. slopes at His right. No one knows what this line means. (Frank Cross and David Noel Freedman once wrote, "Conjectures are almost as numerous as scholars.")
He then explains what he chose to do to create a translation.
Robert Altar leaves part of the verse transliterated but untranslated in his The Five Books of Moses: A Translation with Commentary.
He writes the verse this way:
"The LORD from Sinai came and from Seir He dawned upon them,
He shone from Mount Paran and appeared from Ribeboth-Kodesh, from His right hand, fire-bolts for them...
His translation note reads as follows:
appeared from Ribeboth-Kodesh. It is safest to construe the last term here as an otherwise unattested place-name, though some scholars understand it as "the myriads of Kadesh" or "the myriads of holy ones"...
fire bolts for them. The Hebrew 'eshday, anachronistically construed by later Hebrew exegetes to mean "fire of the law" is not intelligible...
The LXX merely reads:
μυριάσιν Κάδης or myriads of Kades, leaving the Hebrew untranslated, but transliterated.
So, for a final answer, the reason you have such varied translations is simply because scholars cannot agree on how to translate what amounts to a nearly untranslatable text, especially when no one is quite sure what the Hebrew even means.