The problem is, the old OSes often use depricated APIs that are no longer supported in newer operating systems, newer OSes have, generally, improved security, which means things need to be done differently etc. or new browser features require features only found in newer versions of the OS.
It also means extra testing and extra code to maintain to keep it working on older operating systems. What works on one release might crash another, so extra time is required for an ever diminishing market. At some point it just becomes uneconomical to continue. Even (especially) open source projects have limited resources.
Dropping those older OSes means a slimmer product, because you can get rid of a lot of kludges and workarounds, drop code using old APIs that are depricated or don't exist on more modern versions of the host OS etc.