Why do peppers grow poorly in Northern US?

Peppers like warm, mulched, soil (ideally not clay, either, especially as clay insulates and locks in coolness well), and they don't prefer cool nights. Soil can be a lot cooler in the north, as can be the nights. Also, peppers seem to like the sort of consistent moisture that comes with mulching, but I imagine that's not any bigger of an issue in New York than it is further south.

It could be that there's less copper in your soil, if the walls are thinner. Copper helps fruits to absorb water, and in theory, that may lead to thicker walls. I would look to soil nutrients as much as the climate differences.

You can grow peppers in the north just fine, even in less-than-ideal soil. You just need to mulch it and/or warm it up and/or have an excellent soil that peppers love. The only mulch that I have experience enough using to recommend is shredded wood mulch; peppers like it in my garden (its presence or the lack thereof makes or breaks the in-ground harvest, in our soil; they like my neighbor's soil without mulch, though). To warm soil, you could try using such as black plastic or landscaping fabric.

When I don't mulch, I like to grow peppers in containers, but if I have mulch, I like to put it in the containers, too, to reduce watering frequency needs.

In theory, peppers might prefer more phosphorus in the north, due to the colder temperatures, and maybe even more nitrogen to prevent the potassium from dominating (high potassium and cold nights doesn't seem to be the best combination for peppers).

Peppers like heat more than tomatoes do.

You need a greenhouse to grow peppers north of Zone 8 or so!

It need not be expensive or complex. A cloche (single-plant greenhouse) or cold-frame (old window over a raised bed) will do nicely.

Whatever you use, peppers like soil heat. They won't be happy if you can't get the soil over 21°C (70°F) or so. Ideally, you want as high as 30°C (86°F). So a raised bed will help your greenhouse covering to heat the soil better.

Peppers do well in containers! Containers in a cloche or small greenhouse will heat the soil better than in-ground planting. And peppers are actually perennial, so you can bring your containers in the house for the winter this way, too!

You can easily make a cloche with an armature of sticks and some poly film. For watering convenience, make it so you can remove it, rather than poking sticks in the ground. I've seen them made from old thrift-store lamp shades, as well!