Charles Spurgeon Commentary Luke 8:4-6

"And when a great multitude came together, and they of every city resorted unto him, he spake by a parable: The sower went forth to sow his seed: and as he sowed, some fell by the way side; and it was trodden under foot, and the birds of the heaven devoured it. And other fell on the rock; and as soon as it grew, it withered away, because it had no moisture." — Luke 8:4-6 (ASV)

Or, as Mark records it, because it had no depth of earth. There was just a thin layer of soil, enough for the seed's germination and early sprouting; it came up all the more quickly because it was so near the surface and because the heat could reach it so easily, with the hard layer of rock beneath speedily sending the heat up to it. But, for that very reason, as soon as it was sprung up, it withered away, because it lacked moisture.