What is the only major US city without skyscrapers? Why?

Skyscrapers need pretty stable land to sit on. Japan has some large buildings but the Building Codes are very demanding because of earthquakes. Places with a lot of potential for earthquakes wisely try to limit large buildings that could fall down easily. You could mean some place in California.

Sandy or very wet soils are also unstable and so not suitable for large or tall buildings. A lot of engineering goes into creating a building of this type.

Local ordinances also can limit tall buildings as they can block sunlight or views (certain London, England viewpoints are protected by Law) and other local factors can limited the sizes of buildings.

The city I live in has many undocumented coal mine shafts from earlier times that affect the foundations for tall buildings.

Local ordinances limit Building heights.