- Why aren't there clones available for it? And also variations? Such as already-available custom versions of the UNO and Nano, etc made by other companies?
The economics of "cloning" are complex. I would guess that since UnoWiFi2 uses a bunch of new chips (ATmega4809, mEDBG, Crypto Chip, IMU) and modules (UBlox WiFi/BLE), those are not easily and cheaply available in the countries that produce cheap clones. Someone could clone the board, but it would NOT be dramatically cheaper like the Nano derivatives while still maintaining similar functionality, and wouldn't sell very well. (sort-of similar to the Teensy3.2 "clones" (?) you can find on AliExpress - somewhat more expensive than you can get them from US vendors...)
As for variations (loosely speaking):
AVR-IOT WG Development Board (on sale now!) ATmega4808 with WiFi module.
ATmega4809 Xplained Pro
ATmega4809 Curiosity Nano At $10, a cheap "Nano-like" board. Includes debugger chip. No network, though.
All of those will take "some effort" to make worth the Arduino IDE.