Trees and rocks had to be cleared from the land, as there was no use trying to sow seed for a crop on land that already had vegetation. Once the land was cleared, seed still could not be sown without adequate preparation. Plowing and fertilizing first had to be completed. Jesus was teaching that seed will not germinate or grow to maturity when it falls upon unprepared soil. The farmer certainly did not plan to sow seed in thorns or on shallow or rocky soil, as he had prepared the ground to plant it with good seed.
Good seed grows well, and matures and bears fruit only in prepared soil.
However, there are many difficulties in evangelism today. Very few real unbelievers will come to a church meeting. The drop-out rate in regard to those who respond at evangelistic meetings is quite large, although real, lasting conversions do occur. Many commitments made at crusades are rededications, or shallow commitments that don't last. Very few lasting conversions of true unbelievers occur at such rallies.