I have noticed that too. http://goo.gl/w8OehW
Trade Assurance is free of charge and it is fair for suppliers and buyers.
When ordering by Trade Assurance, you send the payment to an Alibaba City Bank in Hong Kong.
Alibaba keps the payment accordng to the contract you settle before.
The supplier needs to invest its own money to produce your order befor recieves a % of the payment.
There are thousands of orders coming up with this metod, usually orders that takes 20-30 days to be produced.
Some suppliers get out of credit or money to produces and they are now rejecting this.
I think there are more reasons from supliers to preffer another payment option.
Alibaba must check and delete the Trade Assurance from the suppliers that can not comply it.
I suggest you to:
1-contact alibaba buyer service and comlaint
2- Dont use Wire transffer, use Letter of Credit instead.
Hope this helps