Tour de France - Polish cyclist Pawel Poljanski's photo
Muscular engagement involves a lot of blood flow to those muscles. The more blood flowing means the more the veins will dilate or bulge.
Also, I'm not claiming to know anything about competitive cycling and I don't mean to throw around baseless accusations but for what it's worth, steroids make you more vascular as well.
You could also mention Angiogenesis, which is the growth of new vascular tissue which is associated with exercise. So extreme cardio athletes like these guys most likely grow more veins and capillaries than a normal person. And you also need to keep bodyfat % in mind. The biggest factor that decides a person's vascularity is how low their body fat is, the less fat there is under the skin, the more veins will show through.
Not only do you have to ride side by side with 100 others for hours on end at 50km/h, while at a physical limit where one slip of concentration can bring everyone down, you have to climb the steepest mountains in the world, and still have enough energy to sprint at 60+km/h in the finale. And that's not even talking about the descents, where they often go in excess of 100km/h just to gain seconds on their rivals. Oh, did I mention they do this day in day out for 3 weeks during the grand tours?
The cynics can say what they want about PED's but the truth is, even with all the drugs in the world none of us would be able to do what they do at their level. The drugs don't miraculously make you teleport to the top of the mountains - they often just increase your threshold during training, so you can train harder and longer and get better results.
I'm in no way condoning the use of drugs, but you can't use that as an excuse as to why these guys can do this. This is one of the hardest professions in the world, where they still need to battle for new contracts each year unless you can deliver under these conditions.
These guys are insane.