We have the same for the tax system in Sweden. The reason there is multi part but two big ones are:
Guarantees around how long time processing your tax information will take. But this gets harder if your information comes in at off hours since the tax information still needs a human stamp of approval (which really is making sure the system didn't flag it as manual review which happens at random and if there are discrepancies)
The second, related one, is that they do batch processing at night and while they could queue data for the next day doing so would require a rewrite of the law guaranteeing a certain processing time, since if your data comes in after the start of the batch run it won't run until the next day, which would be hard to properly inform people about in a way they'll accept. Better then to simply not accept it then.