Because you were stomping around so heavily on the dirt, grass, gravel, and stone that the endermen heard you coming from several chunks away and commenced to pave over all the nearby cave entrances hoping you wouldn't ruin the landscape with your vicious shoveling and pick-mining.
In 1.7 they greatly reduced the "size" of individual cave systems, by a factor of nearly 3, while more than doubling the number of cave systems, leading to an overall decrease of 23% in the number of caves (tunnels), with around half the decrease in total underground air volume due to less dense caves not overlapping as much and ravines being unchanged (ravines are around 25% of the total underground air volume due to caves and ravines). They actually only changed two hard-coded (shame on Mojang for not exposing these in Customized) values in the code but the differences are unmistakable:
A comparison of caves between 1.6.4 and 1.7:
Not only were cave systems larger and denser in 1.6.4, there was also more regional variation in cave density as the aforementioned cave system "size" parameter is actually a range, with smaller sizes being more common; even then a cave system at least as large as the largest possible cave system in 1.7 is over 150 times more common in 1.6.4, such that I find about one every 1-2 play sessions given how much I explore, and the fact that they are so much more common greatly increases the chances that they will be close enough to form larger complexes (nearly all the cave systems seen above are multiple cave systems in adjacent chunks).
Also, here are screenshots from various caves that I've found in my first world (vanilla); many of them are so dense that there are no recognizable tunnels, just a bunch of random-looking formations in their centers:
Also, not as apparent on the two maps above is that mineshafts were made 2.5 times less common in 1.7 and dungeons are more than twice as rare.