That is a fair point, so obviously there are certain layers of objectivity that need to be considered. Number of unsuccessful bids can be one of the criteria, so for example Mexico and Serbia can have an equal chance next time if some new country expresses interest, which hasn't done it for 2017 Open earlier, and 2019 Annual now. Also, user's activity is another thing that can be considered. It's not the same if former hosts are bidding against ever-invested first timers like bestmen or myself, or if they are bidding against a user from Birbistan for example (I don't want to name a specific nation), whose user logs in twice a year.
In any case, here we have the following facts:
1) Mexico and Serbia have both already hosted a TISC edition.
2) Both of them hosted it not by winning, but by bidding or other such means. So in a sense they have already been successful in the process.
3) Mexico and Serbia have hosted TISC editions at their 2nd and 3rd participation, respectively. Algeria and Croatia (with this jury) have both done it 7 times and haven't hosted yet.
4) Neither Mexican or Serbian users are conclusively more active than the other two bidders.