High quality thermals can work great as a day sight, and particularly against warm blooded targets / vehicles / other things that actually put off heat, they can be extremely effective--yes, you can have some difficulties with inert targetry that might require additional inputs to generate the appropriate temperature differentials, and yes, there can be a slightly higher chance of "masking" due to ambient temperature / temperature of the surroundings, however these things can also affect using a thermal at night.
The bigger issue is that thermals are very often not very fast, at least not in comparison to a red dot sight or even 1x LPVO, which can be an issue especially in fast-moving, maneuver and offensive engagements, and honestly and frankly that most people are so much more comfortable and so much more familiar with our "normal" daytime vision that most people tend to prefer it, even in situations where it may be inferior to using a thermal (e.g., camouflaged or dug in enemies at a distance). At night, thermal is awesome, because otherwise you can't see shit [/raycharles.gif] and you don't have to "fight" your own psychology to use it. During the daytime, when you can see perfectly fine, it can be a harder sell. I've seen this on vehicle crews all the time where the sighting systems are often primarily thermal (tanks, CROWS, etc.) but also have a daylight view--gunners will try to use the daylight view and miss or be slow to pick up camouflaged targets against a dense background, however under thermal, those same targets pop right out.
~Augee