Why only humankind will be judged?
If we observe our own creation, the fact is as bright as a daylight that humans are in control of the resources of this planet and make use of them as they wish to. Other creation apart from humans such as animals, plants, birds etc. do not inhabit and make use of the planet as humans. This is the biggest fact that humans are intelligent beings and therefore more worthy of handling this planet on behalf of Allah ﷻ than any other creation of His. This ability of humans is a trust that Allah ﷻ has placed on our shoulders for which we will naturally be accountable. The Quran confirms this for us when Allah ﷻ refers to this burden as Amanah i.e. ‘A Trust’ that He handed over to Adam (alaihi salaam) and his progeny because we have the capacity and intelligence to carry it as opposed to other creation: “Verily, We did offer the trust [of reason and free will] to the heavens, and the earth, and the mountains: but they refused to bear it because they were afraid of it. Yet man took it up, for, verily, he has always been prone to be most wicked, most foolish.”
The scholars of Islam say that the wording in the verse “prone to be most wicked, most foolish” means that the one who has the potential to become wicked can also be just and deliver justice and the one who is foolish and ignorant can also carry knowledge and wisdom. This trust makes us sentient and evolutionary beings by providing us the gift of free will and reason with which we have the ability to progress intellectually and spiritually unlike other creation that is stagnant in its nature devoid of the potential to develop. It is one of a kind blessing Allah ﷻ reminds us so we are grateful: "And had it been Our will [that they should not be free to choose between right and wrong], We could surely have given them a different nature [like animals or plants and trees rooted] in their places (or in their instincts), so that they would not be able to move forward, and could not turn back." Therefore the potential in us to act justly and carry knowledge and wisdom is why humankind were handed this trust. After gifting us with intellect and reason, Allah ﷻ send messengers and prophets to remind and explain exactly how we need to take care of that trust. They also made us wary of what are things that can lead towards our destruction in this world, and if not here then definitely we will have to face it in the hereafter, as Allah ﷻ created this life only to test us. A hadith of Prophet Muhammad ﷺ tells us how the amanah has been distributed among humans in any particular nation. It may shed some light on how each of us will be accountable for what is under their authority and answerable based on their individual capacity to discharge their responsibilities: “Each of you is a shepherd and each of you is responsible for his flock. The amir (president) who is over the people is a shepherd and is responsible for his flock (country); a man is a shepherd in charge of the inhabitants of his household and he is responsible for his flock (family); a woman is a shepherdess in charge of her husband's house and children and she is responsible for them; and a man's slave (or employee) is a shepherd in charge of his master's (or employer’s) property and he is responsible for it. So each of you is a shepherd and each of you is responsible for his flock."
 Al Quran 33:72 – Asad Translation
 Al Quran 36:67 - Asad Translation
 Sunan Abi Dawud 2928