Regardless of region, earth's tilt results in yearly seasonal changes, which organisms deal with through processes like migration/hibernation. Both seem like attempts to avoid the extreme fluctuations... so why haven't certain organisms developed environmentally-reactive bodies, changing with the seasons throughout the year (based on measures like temperature- I.E., additional fur growth during the winter), and why hasn't this continued as an evolutionary advantage to allow organisms to thrive in longer environmental ranges (like staying in place instead of migrating) or even go a step beyond and take advantage of the qualities specific to each season?
As Jamesqf pointed out, food supply was likely a big contributor. As a general expansion onto that, I believe a sufficiently reasonable answer could be that animals likely found solutions (I.e. migration and hibernation) to deal could with seasonal factors out of their control (like lack of resources) before natural selection leading to more defined adaption could/would've taken place. Their behavior dealt with the problem- which leads to another interesting idea that it could have been an early sign of organisms using their intelligence to survive and change the course of natural selection.