Arabic numerals use position to indicate what digits mean. Consider then number "12". The "2" means "two" because it is on the right end of the number. The "1" means "ten" because it is one digit to the left of the rightmost end of the number.
There are two ways to write Arabic numerals: big endian and little endian. English numerals are written big endian. If English numerals were written little endian then twelve would be written "21".
Little endian is better than big endian. Consider the number "123". You start by reading "1". If "1" is little endian then you immediately know what the "1" means. Next you read "2". You know "2" means "twenty" because you are one space away from the start. Lastly you read "3". You know "3" means "three hundred" because you are three digits into the number.
Suppose "123" was written big endian instead. You start by reading "1". You don't know what "1" means so you count the number of digits to the end of the number to figure out that "1" means "one hundred". You cannot read part of the number. Either you read the entire thing or you read it backward, implicitly treating it as little endian. The number is written backward.
Arabic numerals are a clever invention. Whoever invented it was a smart mathematician. Why would a mathematician smart enough to implement Arabic numerals do so right-to-left instead of left-to-right?
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