The Roman way of counting had distinct advantages for simple souls. The great mass of users had only to remember the numerical values of V, X, L, and C, and perhaps occasionally, M. Moreover, it was easier to grasp the concept of a three from III than from the hieroglyphic 3, or eight from VIII than from the complex curlicue we know as 8.
It is generally assumed that the sign for V represents an open hand with five outspread fingers, and that two of these provided X. Originally, it seems, the other three letters used were the Greek chi, theta and phi, which were unnecessary and superfluous in the early Latin alphabet. In due course phi was changed to C for centum, and theta became M, for mille, or a thousand. But no one seems too sure where the sign for 50 came from; perhaps the chi was too much like an X, and so lost an arm or two to evolve into an L.
Be that as it may, this arrangement suited nicely for about 2,000 years. Then around AD 773 a traveller from India arrived at the court of the Caliph of Baghdad with news of an entirely different system that the Hindus had of doing tots. Their numerals included the useful zero concept, and also employed a place value system - the convention that each digit has a significance associated with its position in a multi-digit number. These innovations allowed calculations to be carried out in a way quite impossible with the alphabetical Roman system then in use.
The caliph ordered a treatise to be written on the subject. The task was given to one Abu Ja'far Mohammed ibn-Musa alKhowarizmi, later known by the more convenient name of Algorithmus, and in due course he produced a little booklet which was translated into Latin by one Adelard of Bath around 1120. As Liber Algorismi de Numero Indorum it introduced the Hindu system to the Western world.
As they became aware of their computational advantages, merchants quickly adopted these new numerals for bookkeeping. The system suffered a temporary setback in 1299 when the bankers of Florence banned the Hindu numbers for a time, on the grounds that a zero could be too easily changed into a 6 or 9. But this only lasted for a time; the new numbers gained a wide acceptance very quickly, and the sophisticated computations they allowed helped to facilitate the rise of the bourgeoisie in the Renaissance.
Strange, though, is it not, that the digits so familiar to us now would have been unknown to nearly all our ancestors a mere 600 years ago?