better question may be: why aren't consumers buying desktop apps as much anymore?
1. Low initial investment: Disposable cameras were cheap to buy and so are most SaaS subscriptions, at least in the beginning.
2. Nearly-frictionless termination: Disposable cameras go away when you're done with them and most SaaS can be canceled— and navigated away from quickly.
3. Ease of use: Disposable cameras were simple point-and-shoot devices and most web apps are constrained by the expressibility of WWW UI/UX, so they often cannot easily be made as complex as their desktop counterparts.
4. No maintenance: Disposable cameras were made to be thrown away (at least from the user POV), so users didn't have to worry about protecting and maintaining an e.g. fancy 35mm camera. Similarly, SaaS shifts the burden of maintenance to the company providing the service and is ongoing.
For desktop software…
For #1. You have to install it and, possibly, pay an up-front cost to do so. Furthermore, you have to trust that the software is not malicious.
For #2. You have to uninstall it if you no longer wish to use it and you have to trust that it will clean up its tracks and not leave e.g. broken file associations.
For #3. You have to navigate an UI built under far fewer constraints than UIs designed for the WWW, so you may have to invest time learning its ins and outs.
For #4. You (may) have to keep it up-to-date, usually if you upgrade your OS, but especially if there's a security vulnerability present in the version you're running.
Personally there are loads of desktop apps I want, maybe after getting Beekeeper Studio profitable I'll make some more :-).
Looking at you, Tower
Everything else is pretty much a license key you purchase online to allow you to download/register something you grabbed from the company's website.