Hypothetically speaking, if sliding would get you to the bag faster than running, at what point should you begin your slide? With sprinters, you're just talking about getting a part of your body over an imaginary line. With 1B, you're talking about getting part of your body to touch a specific object on the ground that you must reach and with which you must make contact. I'd have to imagine there's some ideal point in the course of running that would be the time someone should slide instead of run, but if that point is a limited area, I'd imagine it would be hard for someone going into a dead sprint to know exactly when they should attempt a slide into the base. Moreover, in the times I have seen guys slide into first base, umpires have not been forgiving. It could be as I mentioned before, where they lack a good spot from which to view the base and therefore cannot make a good call. On the other hand, sliding into first base is usually done in situations where there is a likely close play at the bag. I wouldn't be surprised if numerous umpires had a slight bias against people sliding into first under those circumstances and would be more likely to call someone out.