TL;DR - The Enterprise was too far away to respond in time
What do we know?
About the Battle of the Binary Stars
- We know it occurred in a binary star system 6 light years from Gamma Hydra (Source)
About the Enterprise
- The Enterprise was under the command of Christopher Pike at the time ('The Menagerie Pt 1', TOS)
- During the course of his command, Pike captained the Enterprise to the Rigel, Vega and Talos systems (Source).
Let's talk about space...
The Gamma Hydra sector was visited by the Enterprise in 2267 in 'The Deadly Years' (TOS) and is near the Neutral Zone established after the Earth-Romulan War (TNG 'The Defector').
In the map of the explored galaxy commonly shown in various episodes (below), the Neutral Zone is to the far left of the image. Now, this map is first seen in Star Trek VI (2293) (Source), so the Enterprise should (barring any strange/unreported instances) have been within the confines of this map. Looking at the three systems it has been, we see Rigel is 3 cubes from the Neutral Zone in a straight line. Since in 'Errand of Mercy' (TOS) was set in Organia and the territory was disputed between the Federation and Klingons, the border was probably closer to the bottom right edge of the map, which would be even further.
We also know that a Constitution class vessel would take two weeks at maximum cruising speed to reach Rigel from Talos (Source), which is travelling five 'squares' on the below map. Add another three 'squares' to travel in a straight line from Rigel to Organia and this increases to just under three weeks.
Another consideration is that the Enterprise's mission was of exploration, so they probably wouldn't have travelled around space at maximum speed, but cruise slowly to explore the galaxy.
What does it all mean?
So, although Talos isn't the farthest point from where I suspect Gamma Hydra is, it just about is. Assuming the Enterprise was at least an average ship in terms of her engines (sorry Scotty), it would take her just under three days to traverse a 'square' on the map. Therefore, all the ships called to the Shenzhou's position must have been inside the 'square' of the Gamma Hydra sector. Since the Enterprise didn't arrive, it mustn't have been anywhere reasonably close.