Following lockdown, many postal and delivery companies struggled to keep up with demand. Not only did they have staff off who may be ill, shielding or isolating but they also had an increase in work load due to the shops being closed and more people ordering online.
For businesses like Amazon, their orders went through the roof and they struggled initially to keep up. They had to prioritise the products that they felt were essentials to ensure that people got them when they needed.
Some retailers simply could not keep up and delays started to drag out in to weeks rather than days.
Over the last couple of months though, more and more of these retailers seem to be catching up and therefore have been able to resume more of a normal service. If ordering online it is still worth reading any information which refers to delivery times as not all retailers are back up to full capacity.
This may be something we have to live with for a while yet, but hopefully over time delivery companies and postal systems will flex to cope with demands as and when they are needed to.