When I learned to fly in the early 80‘s I was told it was done that way to reduce load on the telex network then still used to transmit that stuff.
(I had part of my PPL ground training at an FSS-type facility where the instructor was employed. We got a short break every half hour when he typed the local weather into a teletype which in turn spat it out as a roll of punched paper that was fed into the same machine for transmission to the national weather office.)
BTW, Eastern Europe is on the 00/30 schedule.