The seeds sown among thorns are people who hear the message, but then life's worries and the temptation of money choke the message so that they become fruitless.
Other seeds fell on good soil where they sprouted and grew. They produced a harvest of some thirty, some sixty, and some a hundred times what was planted.
The seeds that fall among thorns are those who hear the message but it's choked out by life's distractions—worries, wealth, pleasure—so they don't produce anything.