There is a theory as to why Hebrew writing runs right-to-left and why Greek runs left-to-right.
The Hebrew alphabet, or more likely the Semitic alphabet of roughly 1500 BC which is the immediate ancestor of Hebrew, was devised for inscriptions to be cut into stone. Now a right-handed stonecutter holds his chisel in his left left and strikes it with the hammer in his right hand. Hence he prefers to cut letters from the right side of a line to the left, so that his left hand (holding the chisel) does not cover up what he has just cut. Hebrew and the other Semitic alphabets, including the Arabic alphabet which is so geographically wide-spead, kept this right-to-left orientation out of tradition, long after stone-cut inscriptions became only a small minority of all writings.
The Greek alphabet, although descended from some stone-cutting Phoenician alphabet, was used from the beginning by scribes writing on papyrus, and thus it was developed as a penman's alphabet. A right-handed penman holds his pen (or stylus or pencil or whatever) in his right hand and, not wishing to cover up or smudge what he has just written, prefers to go from left to right so that his right hand does not rest on or cover up what he has just written. This explains why the boustrophedon system (left to right on one line, then right to left on the next, then left to right again etc.) did not catch on---the slight increase in writing speed from not having to pick up and move the right hand all the way back to the left margin after each line did not make up for the number of lines that were ruined because the penman's right hand, when moving right to left, smudged the ink that had not quite dried when the hand moved on to the next letter.
Latin, Cyrillic, and the other descendants of the Greek alphabet carried on this left-to-right tradition.
This theory makes sense, even though Re: [HM] Orientation of the Number Lineit does not explain why those prolific Roman stonecutters (the ones who carved "U" as "V" Re: [HM] Orientation of the Number Line, a tradition still sometimes seen today on stone carvings) did not rebel and start carving from right to left, or why the scribes who hand-wrote Jewish Torah scrolls never complained that left-to-right made more sense.
What does this have to do with the Orientation of the Number Line? Maybe very little, but it does suggest the question: were the first number lines cut into stone with hammer and chisel, or were they written on papyrus with a pen?
Also another question: most of the world today uses the "Arabic numeral" (more correctly, "Hindu-Arabic numerals") which were acquired by Europe from the Arabs. Numbers in Arabic numerals are invariably written with the most significant digit to the left, not to the right, even by people whose native language and alphabet are Hebrew. (I do not know how Arabs write numbers). Considering that the Arabic alphabet is written right-to-left, did the Arabs write numbers most-significant-digit-to-the-left, and if so, why? If they did not, then did the earliest European advocates of Arabic numerals reverse the orientation so as to match the orientation used in Roman numerals?
Off-topic: if the Hebrew and Arabic alphabets are so closely related, why do they look so different? That is because the ancient rabbis laid down a rule that letters were not to be "ligatured", that is, there were to be no connecting lines between letters. The intent was to make sure that each individual letter was to be carefully formed so as to be recognizable. The developers of the Arabic alphabet had no such rule and went in the other direction, with all the letters in each word to be ligatured together.