The root cause for the removal of such knowledge is lack of its implementation.
Other causes for its disappearance include:
1. Not calling to the knowledge
2. Possessors of knowledge going astray
3. Ignorance of the objectives of Islaamic legislation
4. Obtaining knowledge from the novice and forsaking the proficient
5. Obtaining knowledge from people of innovation and sectarianism
6. Following the Jews, Christians, Magians, and other factions of Kufr
7. Lacking a detailed understanding of evil and its people so as to stay away from them
8. Scarcity of travelling to acquire knowledge after the era of the Salaf
9. Rigid adherence to a specific Fiqh Math-hab
10. Following texts which may not be immediately clear
11. Increase in unnecessary disputing
12. Not contemplating the speech of Allaah and the speech of His Messenger (sallallaahu ‘ahayhi wa sallam)
13. Not consulting the people of knowledge during times of strife
14. Studying without having adequate knowledge of the Arabic language
15. Not learning from those who have sound understanding
16. Obtaining knowledge from print media such as newspapers and magazines
17. Deterring people from the scholars of Ahl as-Sunnah and separating the Muslim masses from them
18. Forgetting knowledge
19. Rarity of acquiring beneficial books
20. Accepting the mistaken idea that Islaamic legislation does not fulfill all needs of mankind
21. Committing sins and acts of disobedience
22. Rulers and heads of state allowing leaders of misguidance to spread their ideologies and mislead people

‏سَمِعْتُ رَسُولَ اللَّهِ ‏ ‏صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ ‏ ‏يَقُولُ ‏ ‏مَنْ نَزَلَ مَنْزِلًا ثُمَّ قَالَ أَعُوذُ بِكَلِمَاتِ اللَّهِ ‏ ‏التَّامَّاتِ ‏ ‏مِنْ شَرِّ مَا خَلَقَ لَمْ يَضُرَّهُ شَيْءٌ حَتَّى يَرْتَحِلَ مِنْ مَنْزِلِهِ ذَلِكَ ‏

…Khawlah bint Hakeem as-Sulamiyyah said:

I heard the Messenger of Allaah (sallallaahu ‘alayhi wa sallam) saying, “When anyone stops at a place and then says ‘I seek refuge in the Perfect Words of Allaah from the evil of what He has created,’ nothing would harm him until he departs from his place of stopping.” [Collected by Muslim]

One of the explanations the scholars have given for the phrase “the Perfect Words of Allaah” is that it refers to the Qur’aan.

In the du’aa above, the Prophet (sallallaahu ‘alayhi wa sallam) directed us to seek refuge in the Perfect Words of Allaah. Since seeking refuge in any created being is an act of shirk, this means that the words of Allaah (including the Qur’aan) are not created because we have been instructed by the Prophet (sallallaahu ‘alayhi wa sallam) to seek refuge in them.

And Allaah knows best.

See: Fath al-Majeed, by ‘Abdur-Rahmaan ibn Hasan, in the chapter entitled من الشرك الاستعاذة بغير الله (Types of Shirk: Seeking Refuge in Other than Allaah); and also Al-Mulakhkhas fee Sharh Kitaab at-Tawheed, by Saalih al-Fawzaan (same chapter).

وَسَامِعُ الْحَدِيثِ بِاقْتِصَارِ …… عَنْ فَهْمِهِ كَمَثَلِ الْحِمَارِ

فَلْيَتَعَرَّفْ ضَعْفَهُ وَصِحَّتَهْ …… وَفِقْهَهُ وَنَحْوَهُ وَلُغَتَهْ

وَمَا بِهِ مِنْ مُشْكِلٍ وَأَسْمَا …… رِجَالِهِ وَمَا حَوَاهُ عِلْمَا

594 Someone who just hears Hadeeth without comprehension is similar to a donkey.

595 Rather, he should carefully study to know if it is da’eef (weak) or  saheeh (authentic), as well as its fiqh (rulings derived from it), nahw (grammar), and lughah (other linguistic aspects),

596 And any contentious issues that may be present, along with the names of its narrators, and the knowledge it contains overall.

A bit of background: They are lines 594-596 from a poem called Nathm ad-Durar fee ‘Ilm al-Athar by Jalaal ad-Deen as-Suyootee, and more commonly called “Alfiyyah as-Suyootee“  which is a thousand-line poem based on the book Muqaddimah Ibn as-Salaah, one of the classical standard references in the field of Mustalah al-Hadeeth.

The similitude of a donkey in the first line was given by Allaah in the Qur’aan: The likeness of those who were entrusted with the responsibility of obeying and practicing the Tawraah, but subsequently failed to do so, is the likeness of a donkey who carries huge burdens of books but understands nothing from them  [See Soorah al-Jumu'ah (62):5].  To this effect, others have written:

إِنَّ الِّذِي يَرْوِي وَلَكِنَّهُ …… يَجْهَلُ مَا يَرْوِي وَمَا يَكْتُبُ

كَصَخْرَةٍ تَنْبَعُ أَمْوَاهُهَا …… تَسْقِي الأَرَاضِي وَهِيَ لاَ تَشْرَبُ

Someone who narrates but does not really know what he narrates and writes,

Is like a rock from which water gushes out and irrigates the lands, while it does not drink any for itself.

Therefore, someone who takes it upon himself to read and listen to Hadeeth must be aware of its level of authenticity, and must also carefully study the Hadeeth in order to learn the rulings derived from it, the grammatical and linguistic nuances it contains, things in the chain or text which may not be readily understood at first, the narrators in the chain of transmission and their reliability, as well as have a holistic view of all the knowledge contained in the Hadeeth and drawn from it as well.

See: Is’aaf Thawee al-Watar bi-Sharh Nathm ad-Durar fee ‘Ilm al-Athar (2/128-130), by  Muhammad ibn al-’Allaamah ‘Alee ibn Aadam al-Athyoobee

I began to see what an important role the rug played in the overall cultural life of the Muslims. Each individual had a small prayer rug, and each man and wife, or large group, had a larger communal rug. These Muslims prayed on their rugs there in the compartment. Then they spread a tablecloth over the rug and ate, so the rug became the dining room. Removing the dishes and cloth, they sat on the rug-a living room. Then they curl up and sleep on the rug-a bedroom. In that compartment, before I was to leave it, it dawned on me for the first time why the fence had paid such a high price for Oriental rugs when I had been a burglar in Boston. It was because so much intricate care was taken to weave fine rugs in countries where rugs were so culturally versatile. Later, in Mecca, I would see yet another use of the rug. When any kind of dispute arose, someone who was respected highly and who was not involved would sit on a rug with the disputers around him, which made the rug a courtroom. In other instances it was a classroom.