The reason is a simple one. In the absence of a pre-approved list, some government body, some decision-maker, is going to have to look at the law, interpret the law, have an application before them—obviously, the patent authorities in all the countries that I'm aware of, in any event—and decide if this is a pharmaceutical product within the definition set out by the WTO, which we've adopted word for word; if it is needed by a country suffering from a public health problem; if it's a developing country, not a least developed one; if that country has established that it has insufficient or no manufacturing capacity; if it is a country that qualifies in the first place; and if it meets the WTO definition.
All of these things require an exercise of some discretion. As soon as you have that, you have the legal basis for challenging them. This is aside from the fact that patent authorities are not the best-placed decision-makers to make calls of that kind. They're not experts on the development status of countries or the public health problems that afflict them. By having a pre-approved list, you avoid those problems and you insulate the decision from litigation to the extent that it is possible to do so.