Vikaram swamy, namaskaram.

1. Alavandar was not a person who would simply go by the external appearance or other factors. He saw the qualities of the person, the devotion, the deepth of knowledge, fortitude, compassion in the heart,  and the ability to present the teachings to others, Therefore he choose Elayaazhwaar.
2. Now your question may be modified as follows: “Why did Perumal make Elayaazhwaar appear in a family with diffrent tradition other than SirVaishnava tradition?”. If Elayaazhwaar had appeared in a family with Sri Vaishnava Tradition, then the future generations would have thought that this philosophy is applicable only to those born in families with such tradition. So, to highlight the fact that the teachings of Alavandhar is universally applicable irrespective of the material circumstanes one is born in, Perumal arranged such that Elayaazhwaar was born in a different tradition. He had the quality of seeking the real Truth and proper discrimination. This is exemplified by the incidence when he gave the correct meaning to the verse mis-explained by Yadava-prakasa. அப்படி பட்டவர், அனைத்து தத்துவ வாதங்களையும் சீர்தூக்கி பார்த்து விஷிஷ்ட அத்வைத்தமே சிறந்தது என்று நிறுவினார். if we also sincerely seek the ultimate truth, we also exercise proper discrimination within the limits of our capability, then we will also conclude that Ramanuja darshanam is the ultimate and strive for the path of Saranagathi.
3. In general we can see that many many many many many…. many great devotees described in our ithihasas and puranas come from such varied background of birth, wealth, tradition and so on. So, to approach Perumal and consider Him as the only goal of life, no material situation can be considered a limitation.

adiyen dasan.