I suspect it's because the old version is uninstalled after the new is installed - taking file associations with it. Been a problem for ages.
Either leave the old version installed (under Advanced Options in the update dialog) - or uninstall the old prior to running the update.
You can try to re-establish file associations in Bridge or Windows, if that doesn't work try an uninstall/reinstall.
Please restart your machine after installing the Photoshop CC also, you can set Photoshop CC as a default program to open jpeg files.
System requirements for Photoshop CC
It would be great if Adobe would add an "Associate image files with Photoshop CC" section in the preferences menu. I can't set PS CC2018 to open any file and have to open them from Photoshop CC 2018 directly. I can't add any programs to the "Default Programs" section of Windows 7 and PS CC 2018 isn't listed there and I am screwed there. When I right-click and select "Open With" and browse to c:\program files\adobe\adobe photoshop CC 2018\ and add the Photoshop CC 2018 .EXE it doesn't appear on the list, even with the "Other programs" tab expanded, so I am screwed there.
The only solution is for Adobe to push out a new update that allows us to choose which image files to associate with Photoshop from WITHIN PHOTOSHOP! Programs like Media Player Classic (freeware) have this option, so I think it's nuts that a premium subscription service doesn't have this option in PS CC 2018!
I am growing increasingly frustrated with this issue and although it's a windows problem, the simplest fix would be if Adobe included the option to set it's file associations itself. All I got from Microsoft support was a copy-paste reply telling me to "Upgrade to Windows 10" which I vow to never do until NO programs will run on Windows 7!
It would be great if Adobe would add an "Associate image files with Photoshop CC" section in the preferences menu.
Actually there is and has been for a long time - in Bridge. But that's not the problem here. The thing is - if you always install and uninstall in version sequence, none of these problems ever occur.
This is a Windows registry issue - but one that should be fairly easy to work around with the Adobe installers. The problem is that the CC installer by default installs the new version first - and then removes any previous versions unless you go in and uncheck that option. Most people don't because they don't even know that option is there. It should be the other way round, and it seems to me that shouldn't be too difficult to achieve.