It should be obvious, but the reason one doesn’t realize the pure Mind, or the same, Buddha-nature, is that it is obstructed by defilements. Ordinary beings (S., prithagjana) are defiled by gross defilements. It’s like a traveler facing a formidable mountain rage he must cross at some risk to his life. It is not an easy crossing. There are powerful urges within such people. They take counsel with many wrong views, opinions, and assumptions. Looking within oneself is of no interest to them. They are often incorrigible and never learn from experience.
Many who enter the temple of Buddhism are content with most of their defilements. They may only want to face and change a few defilements: the ones that are hard for them to live with on a day to day basis such as managing their anger. Fulfilling their strong urges and desires is their raison d'être. What else might be better than this? Don’t say the realization of pure Mind! They are not interested. They only want to continue on their merry way oblivious to what Buddhism is really about; only wishing to get rid of a few defilements.
There is a second—not so bad—group. These are people aspiring to become awakened; who wish to become authentic Bodhisattvas by realizing bodhicittotpada (manifesting the mind that is bodhi or awakened) and thereby enter the path to Buddhahood. This group not only has some measure of good karma from previous lives but their karma enables them to look within; knowing fully well, this is where they will find pure Mind. They have only a few obstructing defilements. Once shown the right course of action, they quickly learn. This group is certainly not incorrigible. They are not facing a mountain range but, instead, rolling hills.
The last group are the Buddhas. They see the pure Mind, clearly. It is like seeing the water the waves are made from being no longer mesmerized by the action of the waves. Why desire waves when you are the water? Thus comes the end of avidya and with it the end of rebirth.
To sum it up, all beings have the potential to become Buddhas because its essence lies in all beings but few can understand that what they cling to and hold dear are defilements. They act to hide the luminosity of this essence or garbha. Every thought, every emotion, including their pains and desires hide it. Despite the words of the Buddha with their simple beauty and clarity they still can’t grasp their import. They twist them into something they are not saying.