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It may be just a smidge to cold where you are for it. It's roots survive, but the top does not. There are a few things you can do. You can give it some winter protection for a few years. Some trees that are right on the edge of a zone they can tolerate can't survive it well when small, but do fine in it when they get larger. I am not sure that tulips trees will do this, but you cold try wrapping it in the winter for the next few years and try.

Another thing you can try is relocating it to an area of the yard that is protected from wind and maybe even try to help along creating a microclimate. Here is more on that:
https://www.gardeningknowhow.com/garden-how-to/projects/how-to-make-a-microclimate.htm

Try neem oil. While you still need to spray the tree, neem oil is actually absorbed by the tree and kills the pests from the inside out. You only need to spray the tree as best you can and the whole tree will then become protected. Here is more information on neem oil:
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

Were the leaves turning yellow before dropping? This would be an indication of root rot. Here is more info on root rot and how to treat it: https://www.gardeningknowhow.com/plant-problems/disease/root-rot-in-garden-plants.htm

Environmental stress can cause leave drop. Dry conditions or over watering.

Also Tulip Poplar is vulnerable to a couple of disease.

Here is a link to refresh you on care.

https://www.gardeningknowhow.com/ornamental/trees/tulip-tree/growing-tulip-trees.htm

You can cut a 4 to 5 " cutting from the tip of a branch.
Remove the lower one-third leaves and snip the remaining leaves in half.
Remove a bit of the bark with a knife on the cut end of the cutting.
Dip the end in rooting hormone and place the cutting in a pot of peat and.
sand mixture.
Maintain a constant level of moisture.

Place the pot in a bright, indirect sunny area that is protected from wind.
Start checking for roots in 4 weeks, but it can take up to 2 months.
Keep the new cutting in a pot until the following spring.
Winter over in a sheltered cold frame or garage with ventilation.

https://www.gardeningknowhow.com/ornamental/trees/tulip-tree/growing-tulip-trees.htm

There could be issues with sunlight or nutrition to the tree.

This article will help you with care information and you can pinpoint the adjustments to make and help you tree to grow.

https://www.gardeningknowhow.com/ornamental/trees/tulip-tree/growing-tulip-trees.htm