Starting with "when" - as already mentioned, it had to happen either on 06.06.2003 (introduction of improved name check) or on 08.04.2008 (changes to Tibia rules regarding names).
As for "why" - Roman numerals are nothing else than capital letters, one or more. The change enforced the usage of capital letters only at the beginning of each word and prohibited the usage of single letters as parts of the names. Why? Let me answer that with the last sentence of one of the old naming rules (which can still be read thanks to the Wayback Machine):
This rule was removed due to automation of the process - instead of allowing the players to make characters such as DeMoNhUnTeR or A B C, then relying on reports and manual verification, such names would be checked and blocked during the character creation, thus relieving some workforce for other tasks. Unfortunately, because of this, names such as Fabian der II or McConner can no longer be used. And yes, the automated verification system could be improved but:
- it would require time and effort, which I doubt CipSoft would find worthwhile
- there is always a risk of an oversight or players finding a loophole if certain combinations were to be allowed (e.g. someone could name their character IV VI IX)
Unfortunately, the system cannot be cheated even by using lower-case L instead of capital I, as another rule (word needs to contain a vowel) blocks that. While it is possible to use a capital I, followed by a lower-case L, the letters are visibly different:
Therefore, if you really want to use Roman numerals in your name, your only choice (at least for now) is to use lower case letters.