Because capacitors are so tiny, there is almost no space at all for marking the value. With through-hole capacitors and hand assembly, you must have the value written on the cap.
So the 2-digits-and-number-of-zeroes system works well on pF and nF capacitors. There's no space to write the p or the n, so the range is continuous, in pF only.
Once you get to uF sized capacitors, there is usually enough space to write the units and more details.
Now that everything is SMD (Yay!) this is less important but the tradition remains.