Why did France refuse to say the word 'Greenwich' for decades?

Because France insisted that the world's zero line of longitude should be neutral, belonging to no single nation, just as it held that the metre was a neutral measure. At the International Meridian Conference in Washington, D.C., in October 1884, delegates from 26 countries voted 22 to 1 to fix the prime meridian at Greenwich. San Domingo, now the Dominican Republic, voted against, and France and Brazil abstained. The practical argument won the day: continuity with most nautical charts, which already used Greenwich.

France held out. It only adopted the Greenwich meridian as the start of its universal day in 1911, and even then it refused to use the name. Its official time became 'Paris mean time, retarded by 9 minutes and 21 seconds', which is Greenwich time described without ever saying Greenwich. France only dropped the phrase in 1978, replacing it with 'Coordinated Universal Time', or UTC.

The push for a single reference came from ships and trains. By the 1870s there was pressure for a world prime meridian for navigation and for unified local times for railway timetables; in the United States, one table showed over 100 local times varying by more than three hours. New Zealand had already set its civil time relative to Greenwich in 1868, and American railways had introduced standard time on 18 November 1883, almost a year before the conference. And in a twist many people get wrong, the 1884 conference never created time zones: it fixed Greenwich as the reference meridian and left standard time outside its business.