METAR you mean? As ATIS is mainly 'event driven' not clock synchro per se (of course METAR changes force an ATIS update)
No idea, but lots of variation around the world on this METAR timing aspect. From on the (half-)/hour, to 5, 10, 15 minutes before and even variations on the minute (as in 9,8,7,... before, for example).
My guess, some historical reason, like the extremely (=as in pre-historic) low baud rate of the telex machines used long time ago in the AFTN communications network. So spreading out the timing, avoiding everybody sending at the same time, maybe??