Hey suntom,
The answer to your question is cut and dry, unfortunately, the reason behind it is not. We definitely know that hypothyroidism effects insulin response as well as how sensitive our cells are to the decreased insulin that's being released. Long story short, we make less insulin and can't even use that little bit very well.
Possible explanations hypothesized to explain this phenomenon span from the dysregulation of mitochondrial oxidative metabolism (the mitochondria are where energy conversion takes place, such as carbs to energy) to the reduction of blood flow in muscle and adipose tissue under hypothyroid conditions (less blood flow, less insulin traveling to where it needs to go).
I hope that helps!
Matt
