People stuck indoors combined with insulin resistance, poor diet, high stress. You only get about 1000 IU's of D3 synthesis per hour for type 3 skin, 2000 IU's for types 1 and 2 skin and that is assuming decent body exposure. It is even lower with type 4 and 5 skin. Most people in that latitude have types 3 and 4 skin. D3 is also measured incorrectly. Medical facilities measure 25-hydroxy which is not showing how much is then converted by the kidneys to 1 25-dihydroxy, the active form of the D3 hormone. People with diabetes and pre-diabetes can't convert the 25 hydroxy as efficiently thus needing higher intake. People with kidney damage can't convert the 25 hydroxy to 1 25-dihydroxy as effeciently. Diabetes and insulin resistance can cause dysfunction in the kidneys. About 80 million Americans are pre-diabetic [1] and about 77% of them don't even know it. Here [2] is some further discussion on that topic which believe it or not, ties into deficiencies in many nutrients but that is a bigger topic for another thread.
[1] - https://www.cdc.gov/media/releases/2017/p0718-diabetes-repor...
[2] - https://youtu.be/5LTWJOi3bCo?t=3 [video] skipping rude intro