We're not immune to drifting away from the gospel simply because we are a fallen race. We are of the race of man whose first father and mother sinned first, causing all of us to be separated from God. That is a generalized answer for it encompasses all of mankind. However, each one of us has certain vices that distract us. It could be a lack of time. It could be overwork. It could be that you just keep yourself busy with family things, or a new car, or a new boat, or a hobby...and so on. We get distracted and if we aren't too careful, we start to drift because we are not spending time in His Word like we should be.
To resist this drifting away, pay close attention to His Words, His commands. Heed them.
In the Parable, they heard the Word but didn't take it to heart. They didn't follow it.