This answer is from the Theravada perspective.
You are thinking about this from the frame of reference of Vedanta, where there is only one thing real and eternal, and the others are illusion (maya) which are not eternal and changing.
In Vedanta, this one thing that is real and eternal is God, whereas you replace this with shunyata (emptiness), which means the great emptiness is the only real and eternal thing, or that the ultimate reality of the universe is voidness.
Next, you say that compassion is the really good emotion compared to others, and this moves one towards emptiness. Then you ask why only such a positive quality moves one towards emptiness, and why not other emotions?
Now, let's change our frame of reference to the Buddha's.
He doesn't ask, "what is the only thing real and eternal?". This was not important to him.
In the Acintita Sutta, he states that metaphysics should not be conjectured, otherwise it will lead to madness. In Parable of the Poisoned Arrow, he says to focus on the immediate problem (i.e. suffering) and not on how it all originated. In the Sabba Sutta, he taught to not speculate about things that's beyond The All.
The Buddha's frame of reference is really the Four Noble Truths - stating that there is suffering, identifying the cause of and solution to suffering, and teaching the Noble Eightfold Path as the method to realize the solution to suffering.
Emptiness in the original teachings of the Buddha refers to all phenomena being empty of a self. This is explained in this answer. This is not metaphysics. It explains that the self is really a mental idea - the illusion, that results in suffering. It doesn't mean that the ultimate reality of the universe is voidness.
It's true that Nirvana is what is attained by the enlightened ones. Maybe you might then say that this is the eternal thing or the ultimate reality of the universe. But Nirvana is just the extinguishment of suffering - it is that which is experienced by the mind when it is completely free of defilements and fetters. It is the only stable source of happiness and peace. It's not any kind of ultimate reality. It's not a supernatural state of consciousness. It's not immortality.
And what about compassion? Compassion is not the only positive mental factor. There are 25 beautiful mental factors listed in the Abhidhamma, in this answer.
Why do positive mental states lead to Nirvana but not negative ones?
That is explained in the Kimattha Sutta - skillful virtues have freedom from remorse as their purpose, and the sutta explains how this leads to Nirvana.
The negative states of mind which are basically combinations of greed or lust, aversion and delusion, that enslave the mind and keep it clinging to sensual pleasures and existence of the self. When one does unskillful things or has unskillful thoughts, this also leads to remorse, and reinforces suffering.
Skillful virtues and positive states of mind brings one freedom from remorse and joy, but this is only one of the three parts of the Noble Eightfold Path called virtue (Right Speech, Right Action, Right Livelihood). There are still the other two parts that need to be cultivated - concentration/ meditation and wisdom.
Without the five precepts and cultivation of virtue, it is impossible to practise concentration/ meditation or deepen wisdom. With the mind becoming positive and virtuous, it can then calm down, concentrate and experience insight and wisdom.
From the perspective of karma, virtues and positive mind also reduces or removes future consequences of bad actions, and therefore reduces suffering.
So, you have to change your frame of reference:
Vedanta talks about the metaphysical reality of the universe (real/ eternal i.e. God vs. the illusion), and it talks about how to go from illusion to real/ eternal. Vedanta also postulates the individual self as being part of, or identical to, God.
Buddhism addresses the problem of suffering and how to end suffering. Buddhism is not concerned with the metaphysical reality of the universe. Buddhism also takes the self as an idea conjured by the mind - which leads to suffering.
From SN 22.86:
In the past, as today, I describe suffering and the cessation of