I'd like to add my comment as an answer to have memory of a side comment.
As I was saying the subtractive notation was a way of sparing characters in carving and this is the reason behind it becoming popular.
@paul garret made a very interesting comment on the fact that with Latin numerals computations were not reported on paper/parchment and computations with the abacus were effective since they did not require keeping track of intermediate steps.
This was, in fact, one of the reasons of a quarrel after Fibonacci introduced Indian digits in his Liber Abbaci (Book of Calculations).
At first computations with what we nowadays call Arabic digits were considered to be cumbersome because they required keeping track of intermediate steps. But, at the time, paper was not widespread in Europe (paper is another invention that arrived in Europe from China through Arabs) and was very expensive.
Abachists kept preferring abacus computations for years since it was cheaper and still effective when compared to computations made by "algoritmists" - such were termed those in favour of arab numerals.
Only after some years, when starting from Fabriano, Italian production of paper became quite standard and costs dropped, algoritmists won their war.
So at least twice in this story the material required for writing numbers played a role.