Why is this? Well, I think there are two reasons.
Here's the first reason:
Foxes don't like to be around humans, at least for the most part. They tend to shy away from us and that's the way it should be.
Since there are less people walking around parks (or other places that I have cameras) during the winter, foxes may be more comfortable with coming out in the day.
More foxes out during the day means more chances of camera-trap images of them in the day.
Here's the other reason:
Winter this year has been cold. Very cold. For the past few days, temperatures have barely been able to make it higher than the 20 degree fahrenheit mark.
These temperatures have been warmer than nighttime temperatures, which have gotten as low as 2 degrees.
Foxes would prefer to be out in the warmer part of each day than out in the middle of the night. This increases the chances of getting red foxes on camera in the daytime as well.