all ISP design a network to be contended. There is never enough bandwidth to go around.
For example if you have 100mbps of bandwidth you might tell 20 100mbps services on it.
This is called contention and would be a contention of 20:1
This is done to save costs as under normal use everyone is not using the connection to the max at the same time.
If you need guaranteed bandwidth this is possible using lease lines but you are paying £1000/month.
Think of the network like the road network. Not everyone road can carry every car at the same time as that would never happen so they are designed to allow a small amount of people at one time.
The lockdown is a unforeseen event that can't be planned for and even if it was planned for the post would mean everyone would be paying leased line costs to support the extra capacity