I am eager to know the reason why did Dronacharya not become a guru of Ekalavya whereas he become a guru of Kauravas and Paandavas".
What is the reason he rejected Ekalavya?
Dronacharya rejected Ekalavya as his disciple because he feared Ekalavya, despite being a Nishada, might surpass Drona's other disciples in archery who belonged to higher castes. He believed only Kshatriyas had the right to excel in archery:
Vaisampayana continued, 'Thereafter Drona began to teach Arjuna the art of fighting on horse-back, on the back of elephants, on car, and on the ground. And the mighty Drona also instructed Arjuna in fighting with the mace, the sword, the lance, the spear, and the dart. And he also instructed him in using many weapons and fighting with many men at the same time. And hearing reports of his skill, kings and princes, desirous of learning the science of arms, flocked to Drona by thousands. Amongst those that came there, O monarch, was a prince named Ekalavya, who was the son of Hiranyadhanus, king of the Nishadas (the lowest of the mixed orders). Drona, however, cognisant of all rules of morality, accepted not the prince as his pupil in archery, seeing that he was a Nishada who might (in time) excel all his high-born pupils. But, O oppressor of all enemies, the Nishada prince, touching Drona's feet with bent head, wended his way into the forest, and there he made a clay-image of Drona, and began to worship it respectfully, as if it was his real preceptor, and practised weapons before it with the most rigid regularity. In consequence of his exceptional reverence for his preceptor and his devotion to his purpose, all the three processes of fixing arrows on the bowstring, aiming, and letting off became very easy for him. (Mahabharata, Adi-Parva)
By the way, according to Narada Smriti, a Nishada is a product of a Kshatriya man with a Shudra woman:
The son called Niṣāda springs from the union of a Kṣatriya with a Śūdra woman. (Narada Smriti, Chapter 12)