Probably because Microsoft is doing less testing that would expose machine or configuration specific problems. The result is upgrades that cause problems for some subset of users.
Also, some of the update techniques Microsoft uses for Windows 10 cause performance issues for some users, but not all. For example, huge amounts of disk activity after an update occurs for some users who have large numbers of files on their disks. Microsoft's update process often triggers a re-indexing of the primary hard disk. The indexer tends to use excess resources if there are large numbers of files on the disk.
If you are just using your PC to read emails, play some games, and do some office work you probably don't have that many files stored on your hard disk. But if you are doing some kinds of work you may well have 250,000 files stored, and see your machine grind to a halt whenever the indexer runs. When you get the install pushed onto your machine mid-day you might as well take the afternoon off.
That makes sense. The first thing I do when I get a new computer is partition the drive. All programs then get loaded on c: and all data goes on d:. Maybe that helps with this issue.