Why are so many people hesitant to update to Windows 10?
People who care about their privacy, security or prefer OSS (Open Source Software) would be hesitant. But for regular user, it'd be application compatibility with W10 and fear the upgrade will break their OS (W7 to W8/W8.1 upgrade was messy)
Well, he said update, meaning go from Windows 7 to Windows 10, so concerns like privacy and OSS aren't likely to be the reason here. My guess is that people just got used to their OS and don't see any good reason to update if what they have works well for them, especially if they have to pay for the upgrade or their system can't handle the requirements of the latest Windows.
why not use linux .ubuntu ,redhat,centos,and so on...
Not all software runs well on Linux, or maybe some people don't have time to learn Linux - there can be many reasons.