RAW is no light
Spells do what they say, and no more. Create Bonfire (Elemental Evil, 152) states:
the magic bonfire fills a 5-foot cube
This is a magic fire, and not a real fire. This magic fire may simply not shed any light, or not enough to be of any mechanical consequence. There are spells that utilize fire AND provide light, but they are a small subset of the fire damage spells and Create Bonfire is not included in them if looking at RAW only.
Comparing against similar spells
Mechanical Consequences may be the guide here. There are a lot of spells that deal fire damage that do not also have a light aspect to them. Some do, like Fire Shield and Produce Flame.
In areas where there is darkness, this is going to make a difference as an extra bonus to the spell that isn't actually there.
If you like at Fire Shield (a 4th level spell), you get both Light and damage.
Produce Flame (a cantrip), gives you Light and 1d8 damage (increasing with levels). Once the spell is used for damage, it ends.
Create Bonfire (a cantrip), gives you the same damage structure of Produce Flame at the same cost, but the trade-off is that Create Bonfire remains after dealing damage. Adding Light does increase the value of Create Bonfire because now you've got the damage of Produce Flame, but you can keep it going for the duration.
Why not allow it to shed light?
At face value, it probably wouldn't do much harm to allow some light, but there is an increase in the spells value if you add functionality to it.
Choosing Cantrips is a cost in itself. You are often limited by the choices you've made and can't swap them out when leveling. If you can get a single cantrip that sheds light and can deliver damage over the duration of the spell,then this increases the value of Create Bonfire.
Yes, you'll need to separate some reality from your game, but aren't we doing that already? :D
Somewhat related is Crawford's ruling on the use Create Bonfire underwater as an example of realistic expectations and D&D:
Create bonfire lasts for its duration, regardless of the environment.