Tyger! Tyger! burning bright
In the forests of the night:
What immortal hand or eye
Could frame thy fearful symmetry?
In what distant deeps or skies
The night is complex, morally corrupt and impedes vision and judgement just as a forest does. Blake may be describing the only environment in which he believes such a fearful creature could exist. Alternatively, he could be implying that the tyger is a shining contrast to the dark forest. 
 Dante uses forest and  feline imagery  similarly in the beginning of  Inferno : 
 
 MIDWAY upon the journey of our life I found myself within a forest dark, For the straightforward pathway had been lost. 
 Ah me! how hard a thing it is to say What was this forest savage, rough, and stern, Which in the very thought renews the fear. 
 
 An alternative view is that the forest is London or other growing cities, places of terrible corruption, suffering and squalor, conditions exacerbated by the beginning of the Industrial Revolution.
The Tyger