Buddha didn't clearly mention the root cause of ignorance. What is the reason? The cause of ignorance is ego (separate self or independent self). And ego was made up by wounded inner child for self-protection from parents because inner child was not accepted unconditionally by parents (unconsciously). The cause of ego is karma till our past life. Karma is imprinted on subconscious and reincarnated. So, the root cause of ignorance is karma. Eventually, the law of causality is complete. On the other hand, Buddhist core philosophy is "emptiness (absence of independent self) or selflessness". In this sense, there is contradiction in Buddhism because self (ego) is recognized as the cause of ignorance even though self is denied in the core philosophy. That's why the root cause of ignorance may not be mentioned clearly.
2. Buddha's real intention
I guess that Buddha recognized ego(separate self） and proposed "emptiness (absence of independent self) or selflessness" as a solution. Buddha must have noticed that most people were ego(separate self）, just like today's world. Buddha understood that the ultimate truth of this planet is "emptiness (interdependence)" but most people had the distorted view which was opposed to the ultimate truth. That's why he said that people suffered from ignorance. Then he showed the right view which matches the ultimate truth as the solution to help them suffer less. I feel like that.
Another possibility is that Buddha wanted to avoid people's misunderstandings because he emphasized selflessness as a solution to transform sufferings too much. If Buddha recognizes the existence of ego (separate self）, people may be confused, because they may have been taught by Buddha that self has never existed before. If so, Buddha can't explain that the root cause of ignorance is separate self (ego).
3. Non-description in Buddhist scriptures
Why Buddha's real intention is not mentioned in Buddhist scriptures? The ultimate truth of "emptiness (absence of independent existence) or selflessness" may have got out of Buddha's control. Therefore, Buddha's disciple may have decided not to mention the root cause of ignorance for avoiding contradictions and confusions even though they knew that it is ego (separate self）. They may not have wanted to go beyond ignorance.
4. Relation with Karma
How can Buddhists explain about the relation between ignorance and karma without ego (separate self）? It is impossible to explain how the imprinted karma on subconscious will reincarnate if we don't use the function of ego (separate self）. That's because ego (separate self） is essential as a means to link ignorance and karma.
5. Revival to original Buddha's teaching
I feel that Buddhism needs to clearly mention that the root cause of ignorance is ego (separate self） in order to revive original Buddha's teaching. Buddha said, "You yourself, as much as anybody in the entire universe, deserve your love and affection". Thích Nhất Hạnh said, "Loving oneself is the foundation of loving the other person". Both of them teach us that self-love is vital. That's because unconditional self-love enables us to revive true self (non-separate self) and release our ego. This will accelerate the evolution of humanity.
A sailboat on the Moray Firth, Scotland Photo by George