Quite simple, it's due to other people using the network.
Like all networks, bandwidth is shared at some point, if someone else's data is being transmitted you have to wait in line. Networks use various ways of managing traffic so you're not waiting forever but it will mean some delay.
As an analogy, imagine you go to the shops. If you walk in at 3PM on Saturday you have to queue for a lot longer than if you walked in at 9PM on Tuesday where you don't have to queue at all. Why doesn't the shop hire thousands of staff and build 200 more checkouts so you never have to queue under any circumstances? Obviously it's not economically viable due to having to hire and train all those staff, it takes up a large amount of expensive floor space, and so on.
In a well run shop it's managed so there's not much queuing during peak hours, this gets customers in and out efficiently and makes them happy. In a badly run shop it can't cope so people get poor service.
This is the reason why some networks are better - they have more metaphorical car parking space, more checkouts, better staff, more efficient layouts, and so on.
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