Originally Posted by Jamesy
Couple of differing reasons there but both seem to suggest that it was due to Damon taking a place from a former champ. So it was his first year of competing in F1 not later as i thought.
This one is from wikipedia.
When Mansell's team-mate Riccardo Patrese left Williams to drive for Benetton in 1993, Hill was unexpectedly promoted to the race team alongside triple world champion Alain Prost ahead of more experienced candidates such as Martin Brundle and Mika Häkkinen. Traditionally, the reigning driver's world champion carries the number '1' on his car and his team-mate takes the number '2'. Because Mansell, the 1992 champion, was not racing in Formula One in 1993, his Williams team were given numbers '0' and '2'. As the junior partner to Prost, Hill took '0', the second man in Formula One history to do so, after Jody Scheckter .