FNB will not explain why they will not use my investments as a guarantee for my credit card limit

I have had a credit card limit of R 80 000 for over 20 years with FNB. I have substantial amounts invested in fixed deposits at FNB from where I receive a good income. I use my FNB credit card for most of my needs including overseas trips, business class travel, and cruises. FNB will not explain to me why they are not allowing me back to a credit limit of R80 000 after I had reduced the amount to R 18000 in November 2023.