There are factors:
1 - Like poor Wifi connection
2 - Slow wifi connection
3 - Too many other people downloading at the same time and Apple Severs are overloaded
4 - Software on the computer interfering with the download process including AntiVirus Software
Notation: Disabling the AV Software may or may not solve the issue. Removal of the AV Software may increase the chances of the download to perform as expected.
5 - Running behind a VPN which may increase Packet losses or corruption of the download
6 - If via wifi too many Other devices using the wifi at the same time reducing the Bandwidth to your computer.
7 - Best Results are via Ethernet Cable to Router