Ha, you got me started; you'll be sorry.
1. Mojave is the last Mac OS to support both 32-bit and 64-bit applications.
2. Mojave is the last Mac OS to allow use of the "old" HFS+ file system, which I prefer to APFS for a number of reasons.
3. Mojave has a 6.3 GByte storage footprint, compared to Catalina, which is 8 GB+, Big Sur, which is 12 GB+, etc.
4. Mojave allows Carbon Copy Cloner to make complete, bootable copies of both data and system files. Because later MacOS versions, which use APFS, separate the System file as "Read-only," and now require "signed" OS files, that is no longer possible.
5. Due to item 2. above, Mojave is the last MacOS to allow efficient use of my (rather expensive) Samsung 970 Pro 1 TB Nvme SSDs, which choke on APFS. They boot under Mojave in 19 seconds; under Big Sur in 59 seconds (if I wanted a complete TRIM).
Other reasons escape me, at the moment.