Because 32bit only hardware was last released 10 years ago. You can not buy a 32bit system today, because even the worst cpus are 64bit capable.

And this impacts the development of the apps and operating systems as well. There are distros which have completely dropped their 32bit editions over the years (arch, elementary os, sabayon, etc) and others shich did not even bother releasing a 32bit edition (solus, clear, libreelec etc). And the same applies for the (mainly closed source) apps,, because maintaining a 32bit version requires resources and time if you want to keep them stable and secure.