From wikipedia, so may be of dubious accuracy, but the following sounds about right
"The first serious attempt by the UCI to introduce compulsory helmet use in 1991 was met with strong opposition from the riders.[sup]
[25][/sup] An attempt to enforce the rule at the 1991
Paris–Nice race resulted in a riders' strike, forcing the UCI to abandon the idea. While voluntary helmet use in professional ranks rose somewhat in the 1990s, the turning point in helmet policy was the March 2003 death of Kazakh
Andrei Kivilev. The new rules were introduced on May 5, 2003,[sup]
[26][/sup] with the
2003 Giro d'Italia being the first major race affected. The 2003 rules allowed for discarding the helmets during final climbs of at least 5 kilometres in length;[sup]
[27][/sup] subsequent revisions made helmet use mandatory at all times."
from
http://en.wikipedia.org/wiki/Bicycle_helmet
There's some quotes here from the UCI in 2003:
http://www.bikebiz.com/news/read/uci-urges-pro-riders-to-don-helmets/03746
So it sounds like on the second attempt there was less resistance because there was already some degree of helmet-wearing going on.
It's probably worth pointing out that - as I understand it - what the UCI says, goes, without a lot of concerted opposition.