Note: this answer is based on assumptions and historical knowledge, but I can't say with certainty it's correct because I don't remember CipSoft ever explaining it.
The element Ice was added to Tibia much later than its other classic counterparts, in version 8.0 when Svargrond was implemented. Before that, even though the Carlin Ice Islands already existed, as well as Frost Trolls, the damage type did not exist. The energy, fire and poison fields (and corresponding damage types) already existed from the early days of Tibia.
At that time when Ice damage was implemented and for a long time after that it was considerably stronger than other elements. Most of the stronger creatures of the game, such as Demons and Hellhounds, were weak to or at least neutral to Ice. Even weaker creatures such as Dragons and Dragon Lords were still considered dangerous foes (and weak to Ice damage).
If an "Ice Field" had been created it would have been too strong if it was used in a way that made sense and blocked the path of some of these creatures, which is probably why CipSoft decided not to create it. In fact, there's no "Ice Field" at all, not even created by monsters or by default on the map.
One could argue that they could have created it later when elements became more balanced and perhaps Ice fields wouldn't be so strong. But then again, everything was already balanced regarding the use of the 3 existing fields (and players used to them) and adding another field would still be a lot of work for them to do.
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