Ryanairitis!
You could have unfortunately been under the effect of a boot of "Ryanairitis"! Malta is one of the countries that doesn't charge for the fire service being on standby, however limits the number of simultaneous requests for this service. Probably you would have got permission if you waited a few minutes.
I suspect you request coincided with some of Ryanair's, who have a blanket approval for refueling with pax on board in Malta, whilst other airlines, including the national carrier have to seek permission each time, which, believe it or not is given subject to Ryanair's requirements.
Last month there was an absurd incident whereby an Air Malta flight applied and was given the go ahead to board while refueling, and just as the passenger coach arrived shipside, the permission was withdrawn as a Ryanair flight had in the meantime used its blanket permission! This is 32 degrees OAT!
Absurd - and hence my belief that these limitations are having commercial connotations, and I wonder about their necessity in today's high-tech scenario.