Because then theymos has to pay more. It could entirely be about costs and whether the forum pulls in enough ad revenue to continue to pay those moderators at the rates that they have been paid.
It's not public information, but mod payments are based on some activity which is unknown even to mods, and possibly based on ad revenue. How they are related, I don't know (if ad revenue is low this month, does that mean mod payments are low this month too? For the same amount of work, do mods get paid less?)
Based on what is known, mods get paid if they do something. Mods who are absent or on vacation, don't get paid.
What will happen between global mods is that the activity that could have been done by one, can be done by the other, and they get the corresponding points or activity, which determines their payment. The more active global mod gets paid more.
If we suddenly have more than one global mod, the "shares" of their payment will get assigned proportional to the amount of work they do. If two global mods are promoted, and one acts as if he did not get promoted (remains in his original or local mod section), then he gets paid as if he did not get promoted.
Previously, (some months ago, maybe a couple years ago), mods were not paid. There is no promise to continue payment, nor to increase, nor to decrease, we don't even know what metrics are used to determine payments. It wouldn't be a good idea to stop or lessen payments now though.
Comparing to other industries, mods are paid "straight commission". There is no "minimum wage". And for most mods, depending on what country they live in, they don't even get minimum wage for being a mod.