Because you're supposed to first uninstall the old one, to avoid situations where there are files for old versions still in the directory. I've personally run into issues with the old method, where the operating system would not allow a particular DLL to be replaced, so MakeMKV didn't work after the install.
The MakeMKV program directory should not contain anything not current.
Several years ago, the things users might edit (like profiles) were moved to the MakeMKV data directory (set in Preferences->General) so they would not be overwritten by an update.
Doesn't it prompt you to uninstall the old version though? It's possible the uninstaller failed to remove some remnant of the old version causing this error.
Just installed 1.14.3, the uninstall dialog removes the old version and no proelm to install the new version.
Good Luck
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Useful MakeMKV links: FAQs - Debug Log - Buy - Expiration of beta key
Two Blu-ray (UHD) Drives LG LG BH16NS55 with Libredrive Firmware 1.04