One major reason is, the orders system would be affect by different game speeds in various undesirable ways. For example, it would effectively be much more expensive to build improvements if a "Marathon" farm took 8 turns to build. That would mean 8 less unit moves, which is a big sacrifice.
But the counterpart is that unit movement is effectively cheaper on slower game speed. If city production and tech takes twice the time to complete, that means units effectively move twice as fast in relative pace. And because orders are 1/turn, this also means you get twice the number of orders in the time a city produces something or a tech completes.
In fact, the increased order cost for improvements gets canceled out completely.
Let's say I'm sitting at a static 5 orders per turn, and I need 4 turns to build an improvement, complete something in my city, or research a tech - all the same for simplicity's sake - and I have 2 warriors and 1 worker. The worker is building an improvement. Across the four turns the worker needs to build that improvement, my two warriors get to use two orders each, for a total of eight orders. Say this is what they require to move across my empire.
Now, we create a x2 game speed. So, it takes 8 turns to build an improvement, complete something in my city, or research a tech. Thus, my worker is consuming an order every turn for 8 turns. That means each of those 8 turns I have 4 orders for other things. My two warriors each get 2 order per turn for 8 turns, and in the time it takes to build that improvement, complete a production in my city, or research, a tech, they can move all the way across my empire... and all the way to the other side again.
This behavior is 100% identical to slower game speeds in Civilization. Orders are a per-turn resource, but they are no different from units getting to move once, workers removing 1 turn from their "turns until finished", or anything else like that. The only difference is that the combined pool of units moving, workers spending a turn building an improvement, units upgrading and so on has an additional limit separate from the individual limits - but still a per-turn limit just like those others.
(side note: I don't actually care about slower game speeds all that much, the game speed as-is feels fine to me, I'm just making this argument because I'm pretty sure it's
possible, and other people might enjoy it)