What was the biggest contributor? Workout regimens have surely played a role as players are more dedicated to fitness and strength training, but Gary Player may have been the biggest workout fanatic in the history of the tour, and he never averaged 300+ yards off the tee.
The obvious answer is advancement in equipment.
In 1980, players were using 43.5" persimmon drivers with steel shafts and balata balls. Todays drivers heads are nearly twice the size of persimmon, made of titanium, and have 45" graphite shafts. Not to mention the technology of today's multi-layer balls.
What advancement in equipment had the biggest impact?
There were two major advancements that contributed to the biggest change. And they both occured within a 4 year span (2000-2003). The first was the introduction of the multi-layer ball (Titleist Pro V1) and the second was the increase in driver head size to over 400cc. The combination of these increased the median driving distance 5% or over 13 yards!
The increase in distance impacted players equally across the board. Corey Pavin has always been one of the shorter hitters on Tour (and happened to be one of my favorite golfers growing up). His average drive in 2000 was 258.1 yards. Just 4 seasons later in 2003 it jumped to 268.9 yards (an increase of 4.2%). At the other end of the spectrum is John Daly. Always one of the longest hitters on Tour, Daly went from 301.4 yards in 2000 to 314.3 yards in 2003 (an increase of 4.3%).