Why is it that the psychological forces like lust, greed, desire, unskilful emotions, etc, which take us away from Nibbana feel so strong and have greater pull than the forces that propel us towards Nibbana like metta, vipassana (insight), etc?
It's extremely difficult to let go of sensual enjoyment. Experiencing sensual enjoyment leads to clinging, meaning trying to experience even more sensual enjoyment.
Hence, the masses have the natural tendency towards burning with sensual fever, rather than trying to escape it. It's a vicious cycle.
From Magandiya Sutta:
"Now suppose that there was a leper covered with sores & infections, devoured by worms, picking the scabs off the openings of his wounds with his nails, cauterizing his body over a pit of glowing embers. The more he cauterized his body over the pit of glowing embers, the more disgusting, foul-smelling, & putrid the openings of his wounds would become, and yet he would feel a modicum of enjoyment & satisfaction because of the itchiness of his wounds. In the same way, beings not free from passion for sensual pleasures — devoured by sensual craving, burning with sensual fever — indulge in sensual pleasures. The more they indulge in sensual pleasures, the more their sensual craving increases and the more they burn with sensual fever, and yet they feel a modicum of enjoyment & satisfaction dependent on the five strings of sensuality.
There's also another vicious cycle.
According to MN 9 (below), as long as you still have fermentation or effluents, you would still have ignorance, and vice versa (also see this question). Ignorance is a very deeply ingrained and deeply lingering fetter.
From the origination of fermentation comes the origination of ignorance. From the cessation of fermentation comes the cessation of ignorance. .....
From the origination of ignorance comes the origination of fermentation. From the cessation of ignorance comes the cessation of fermentation.
There is a vicious cycle between ignorance and fermentation.
It's very hard to escape these. Natural evolution sustains these vicious cycles.
From Iti 109 below, we see that craving follows the flow of nature, and renunciation is striving in the opposite direction.
This was said by the Blessed One, said by the Arahant, so I have heard: "Suppose a man was being carried along by the flow of a river, lovely & alluring. And then another man with good eyesight, standing on the bank, on seeing him would say: 'My good man, even though you are being carried along by the flow of a river, lovely & alluring, further down from here is a pool with waves & whirlpools, with monsters & demons. On reaching that pool you will suffer death or death-like pain.' Then the first man, on hearing the words of the second man, would make an effort with his hands & feet to go against the flow.
"I have given you this simile to illustrate a meaning. The meaning is this: the flow of the river stands for craving. Lovely & alluring stands for the six internal sense-media. The pool further down stands for the five lower fetters. The waves stand for anger & distress. The whirlpools stand for the five strings of sensuality. The monsters & demons stand for the opposite sex. Against the flow stands for renunciation. Making an effort with hands & feet stands for the arousing of persistence. The man with good eyesight standing on the bank stands for the Tathagata, worthy & rightly self-awakened."
Instinct (refer to AN 7.11). The Earth today has 7.7 billion people plus zillions of animals, insects, fish & other life forms. This whole Earth is created by lust & craving (refer to SN 12.44). Basically, each creature is born from reproduction for the purpose to further engage in reproduction. It goes on & on like this, endlessly for the unenlightened (refer to SN 15.1), until the Earth burns up.
To add to other answers, drawing more from Darwin's theory of the evolution : take 2 people, one for which the forces that propel towards Nibbana are stronger, and the other for which craving for sensual enjoyment is stronger.
Which is the one that is gonna reproduce more ?
Obviously the second one. He would be less happy than the first, but from an evolutionary point of view, more successful.