Because of security considerations.
This has been Skype policy since inception and won't be changing anytime soon.
I am afraid that you are incorrect Norman -- I have just had a online chat with Skype support and have successfully changed my PayPal account to the one that is now active. I have also purchased credit with the updated PayPal account details.
Chat transcript below:Irrelevant portions deletedkensalign says
Initial Question/Comment: Credit card payments
7:40:06 PM System says
Thank you for contacting Skype Customer Support!
7:40:06 PM System says
Erzsébet - Skype Support has joined this session!
7:40:06 PM System says
Connected with Erzsébet - Skype Support
7:40:25 PM Erzsébet - Skype Support says
Hello! Welcome to Skype Live Support! My name is Erzsi. How may I help you?
7:43:48 PM kensalign says
Alternatively, I cannot use PayPal as the account connected to this Skype account is no longer valid. I have been told that I cannot change the PayPal account that is associated with this account, so that is not an option either....
7:44:52 PM Erzsébet - Skype Support says
You can change or link as many PayPal account to one Skype username as you wish.
7:45:03 PM kensalign says
How?
7:45:57 PM kensalign says
When I went to pay for Skype credit it used the account on file, with no option to enter the email address associated with the PP account.
7:46:13 PM Erzsébet - Skype Support says
You have to cancel the old Billing Agreement on your PayPal account.
The rest of the conversation is not related to this issue ... but once the old Billing Agreement was canceled, I could then choose PayPal as a payment option. When doing so, I was presented a log in screen to log in to PayPal and made the payment using the different account.