There is a logistic reason for their current plan. But as it is more towards their own stability issues, I could understand why people would not see it.
By offering a short window, they can clear out servers that are currently full (with not only entire free companies potentially moving, but with people who missed the first move, as well as people who just want a smaller server or to get away from people) to less crowded servers. Giving way to people being able to log into the servers that get spaced freed up.
Where, providing a more long-term movement window, there is the issue of slots only opening up in small bits, causing pains for people who want to move to full servers would could potentially bump up against server full "walls".
Either way provides a level of pain, both offer revenue options for SE, and both have the potential of seeing the same pain you are seeing now.
When you are planning for budgeted server allotments you will see issues like this if things are going well. The only time you don't see problems like this is when things are not going well.
Of course, they could always just throw more servers at it, and then attempt to resell them when things die down, though that is not only an uncommon business practice, most people would be laughed at in the business world for even suggesting something like that.
Either way, the issues will clear up in a day or two, and in two weeks it will just be another bump in the road.