No-one needed it, or wanted it particularly other software vendors who had to do all the additional work for compatibility, and for almost NO new product sales. Imho, Catalina is the OS equivalent of 'New Coke'. In many cases, such exploitable coding remains into further builds of the OS until reported, patched and then disclosed by the software vendor. A (within reason) older, more tested, and well patched OS is generally more secure because the more easily identified and remotely exploitable coding errors are found 'quickly' upon release, and then patched. These new OS builds always contain more interesting things to find during initial auditing, and always make them much more targeted/given additional scrutiny. Newer OS do introduce new code, which has been tested by MS or Apple however, it has not been exposed to the scrutiny of the white/grey hat penetration testers/bughunters and outright hackers. Security is another issue, and a bit more complicated. No-one has yet suggested it runs anything any faster or more efficiently, and actually displayed convincing results of this.

After that, you can hear the winds whispering.

I have asked this often about Catalina, and I only get one reply which most agree is a nice new feature. instead, the newer versions often just add more bloat and of course, they do sometimes introduce very useful and great new things. but it's not usually the case where Apple or MS are re-writing massive sections of the OS coding. I know this can be achieved by the programmers writing more optimized and efficient code. it's not common for new operating systems to run much faster using the same hardware.

Upgrade high sierra to catalina