I hope this can be answered before GB start.
Why neither CannonKeys nor iLuMKB can not be paid by PayPal?
I tried to go through their check out process and I found out that I can only pay by credit card at stage of payment.
Or this incoming GB is an exception and can be paid through PayPal?
It's not an exception.
Paypal recently changed their policy - in cases of any refunds, they used to refund the 3% processing fee, but in Sept 2019, they announced that they would no longer refund the 3% fee on returns.
Instead of having to raise prices to cover the added costs of refunds, we instead decided to drop PayPal for implementing this very anti-consumer move.
There are many alternatives:
Credit cards have a lot of safety features, many of which are better than PayPal
Apple Pay and Google Pay are more efficient than PayPal, and you can use Paypal through Google Pay.
If you want to blame anyone for this change, blame PayPal