Sri Venkateshwara was not an incarnation of Mahavishnu, he was Mahavishnu incarnate. As his Mangalacharan states
श्री वैकुण्ठ विरक्ताय स्वामि पुष्करिणी तटे |
रमया रममाणाय वेन्कटेशाय मङ्गलम् ||
Getting displeased or bored f Shri Vaikunta, the lord came to the shores of Swami Pushkarani (the temple tank) where he frolics with His Consort.
Sri Venkatesh Mahatmyam narrates why Mahavishnu came down to Earth. At the beginning of Kali Yuga the great sages had gathered at Naimisharanya to perform a great Yagya for the benefit of humans. There was a debate as to who should be the Yagya Purusha he main deity, and it fell on the learned but arrogant sage Brighu. By his austerities Brighu had eyes in the sole of his feet.
Brihju first went to Satya Loka. Brahma was not there and the arrogant sage sat on Brahma's throne. When Brahma came he was furious and cursed Brighu. In turn Brighu cursed Brahma that in Kali Yuga there will not be any temples for your worship. In fact the only templf e where Brahma is worshiped is at Pushkar in Rajasthan.
Brighu the went to Kailasa. Shiva and Parvati were love making - and Nandi stopped Brighu. An angry Brighu stormed into the inner chambers and cursed Shiva that since you are always only interested in lovemaking, no temple of yours will have your image. You will be worshipped in the form of a Linga.
He then went to Vaikunta where Vishnu was sleeping. Brighu kicked Vishnu on His chest. Vishnu woke up - O maharishi Brighu, my chest is like hardened rock, but your feet must be hurting. Massaging Brighu's feet Vishnu pulled out the eyes in the soles of his feet. Immediately he lost his arrogance, angered beyond sought Vishnu's forgiveness and declared him to be the Yagya Purusha.
But Lakshmi was angry and she quarreled with Vishnu - you no longer love me - here a good for nothing Brahmin kicks you on your chest where I reside and instead of punishing him you massage his feet and seek his forgiveness. You don't deserve me and so saying Lakshmi left Vaikunta. she came to the Venkatadri Hills and started living as a cowherdess. Without Lakshmi Vaikunta lost its lusture and charm and so following her and searching for her Srinivasa came to Venkatadri Hills. [Some of these aspects are sourced the TTD publication on the Story of Seven Hills]