Because as I understand it
the company involved has an SOP that if "full time" auto thrust/autothrottle is available it must be used....the rule applies to almost all types, not just Airbus. That said you can despatch with autothrottle/auto thrust inop as per the DDG and the FCOM, you're obviously (?) trained and checked on in the sim on flying with manual thrust, but you can't opt to fly manual thrust on the line simply because you want to....
IMHO and that of many others for somewhat obvious reasons it's a somewhat a flawed SOP !!!!!! and has been the subject of much heated debate elsewhere, but ultimately management and the lawyers write the Ops manual. I wonder if it will now be amended.....
And if you must know I haven't operated my type on the line using manual thrust for well over five years...........