Only a few centuries ago people had no idea about hygiene. There was simply no idea that dirty hands, filthy surfaces etc. was a good environment for growth of bacteria causing diseases.
Only a century or two ago (depending on country) there were no public schools. There was no common understanding that children require systematic education in language, mathematics, and sciences.
Only 50 years ago there was no notion of healthy lifestyle. The virtue of healthy natural diet and exercise was virtually unknown to the wide population.
Only 30 years ago the scale of human impact on planet's ecology was not understood at all. Humans were completely careless about pollution and overconsumption of natural resources.
Similarly, the real science of sentient ethics is not yet understood by humanity. There is no understanding of conceptual mind, how it develops, how it builds its picture of the world and the self, how this picture leads to actions, and how this creates experience of happiness or suffering.
Happiness is not really understood. Suffering is not really understood. Mind is not really understood. What's good and what's bad is not really understood.
Since people don't understand happiness, suffering, mind - they are confused about what is really important. They get fooled by the fake shiny promises of the materialistic civilization. They pursue things they were told will make them happy. But as they do that they only end up creating even more trouble for themselves because they don't understand how things work. They keep spinning in this vicious cycle of trouble, confusion, fake promises of happiness, and wrong action causing more trouble. They think they have no time to stop and look around. They don't know any other way to live. It really is very sad.
At some point humanity will evolve to understand these concepts, and then the things that Buddha taught will be common knowledge taken for granted, just like hygiene, public education, healthy lifestyle, and ecology are taken for granted today.
The question you asked is Why. Why are people ignorant of how mind works? Because self-reflection and self-awareness is hard. It's much easier to study the external world. Most people don't have enough motivation to study the external, let alone the internal which is much harder to understand.
Why don't people have motivation? Because they don't see the reward, they don't see how studying mind can help them achieve their goals.
Perhaps they have wrong goals you'll say - and I will agree, most people pursue all kinds of superficial goals they adopt out of ignorance.
As Buddha explained, ignorance is innate. The mind is a recognition and interpretation machine. It recognizes signs and interprets them to their meanings. The tremendously complex system of signs and their meanings is our model of the world, model of reality. This model is rebuilt by each generation based on the information passed on by the previous generations. The way this model is constructed starts from the superficial observations and gradually moves to deeper realizations.
Children are superficial, their minds are superficial, their understanding of life is superficial - because they are just starting out. Similarly, most people are superficial about mind, and happiness vs suffering, and ethics (what goals are worth pursuing). Why? Because it's deep, it's not obvious, it's not easy to see. And because other things that are easy to see completely absorb their attention.