This understanding is not a matter left to human intelligence, it is plain and evident to persons of all IQ's/literacy/backgrounds. Satan is not omnipresent so he utilizes man's sheep like pack and conforming nature. He will twist and distort ones intentions and honest curiosity, even God's own words to produce doubt and apparent contradiction to foster rejection and rebellion. It is not understood because the heart has fattened and calloused against it. A mans own peers become the fowls of the air as much as any demon.
From whom does the most pressure/persecution come from? Those just described as the wayside. Those here now without root expect peace and pleasantry. If the gospel is obvious to them it should be obvious to others they presume, and yet it is not obvious, ridiculed, divisive, persecuted against, even deadly. The young fledgling becomes disheartened, embarrassed, offended. A process - sunlight that is supposed to bring about growth instead kills off the plant. Though passive from ther out typically by number they are used by the wayside (therefore Satan) to increase the pressure on the remaining believers.
A believer that overcomes the first two elements must also contend with the cares of this world. It may not matter what others believe or think or behave, the believer is settled and assured in this part of his faith. However, the real issues of life and family and citizen present a constant drain on his time and energies causing anxiety and fatigue and over extension. The productive fruit of his life gets chocked out without him barely realizing it. I speak from experience.
For the few strong that remain all of this process works to produce fruit in them 30 60 100 fold; supernatural returns. Fruit like this would not be possible if it were not for the entire process the necessitated it and brought it about. We should not consider it odd that these many things go into producing spiritual fruit. It is producers like these that the Lord wants with Him in His Kingdom. These souls are basically the humble and meek He spoke of in the Beatitudes that His light and Spirit have shown through, His redemption has fashioned, branches abiding in His vine, created unto good works; they are the blessed.
The Lord sows a very particular seed expecting later a very particular fruit, a fruit worthy of the kingdom from which it was born, worthy of acceptance into the kingdom to which it is harvested. It is a supernatural fruit that comes forth in supernatural numbers; the fruit itself that bares more of this particular seed. The process it takes to produce from seed to root to a plant to fruit is deliberate and unavoidable as sunshine. Understanding this parable is to understand such spiritual process. Is this the fruit that I am producing?
This parable is to be placed beside the parable of the sower for direct comparison, one extends the other. The first is a look at the process of the individual believer and what he must overcome. The second is a picture of the field (world) of all. The enemy has come in and planted a false believer with every appearance of the true believer except in final fruit. To remove the false believer at this time would also uproot the true believer, so the two are left to grow together. Both parables work together to draw a broader and deeper picture. The Disciples will shortly ask Jesus to explain this further.
This is not to say that the Devil has created his own humans and planted them amongst God's. This is better explained by whom the God created human becomes moral agent of/to. The Kingdom of Heaven becomes firmly planted into one and the false kingdom of heaven becomes established in the other. The differences nearly indistinguishable until the fruit from each is bore out.