Trying to gain a full evolutionary answer from a monospecific species isn't an easy task. Also, making a bold statement that fish had eyes on the sides of their head is a misdirection since there are (and were) numerous species with eyes on top of their head. As well as species with dorso-ventrally compressed cranial structures. As far as having a neck, there are species of fish that have developed adaptations in their musculoskeletal archetecture to allow them to make movements that other species cannot. Polypterids are a quick example of a species adapting a left and right head movement.
The question itself is written too broadly to know where to begin such a discussion. Many other evolutionary links were being created at the same time in numerous different niches and involving countless other species. Add to that the likelihood that tiktaalik may be in the company of thousands of other species that proved unsuccessful in their adaptations giving them an advantage over another developing genus and, therefore, their line ended. Much like the lines that developed the Rutiodons.