One has 327 million people and an area of 9.8 million square kilometers (36 people per Km2). The other has 8.4 million people and 41 thousand square kilometers (216 people per Km2).
Switzerland has one of the highest and fastest internet penetration rates in Europe. If you rent an apartment, cable is often thrown in as part of the cost. The rest of the people's landlines are ADSL (which isn't very fast unless you use the latest ADSL2 dslams), or VDSL, which you can only get if you're very close to a CO. Finally, fiber is easier to roll out when you have a smaller population in a smaller area.
Most Americans cannot get VDSL, and the DSLAMs are old. Cable is pretty fast, and fiber is slowly getting rolled out, but it's a big freaking country, meaning this is expensive as hell.
The mobile providers in Switzerland only have to provide a very small number of broadband equipment and mobile towers compared to that needed to cover the US. Also, the same equipment used in Switzerland covers up to 6x more subscribers, because of increased population density.
The US has to provide much more equipment and installation over a much larger area to cover a comparatively smaller number of subscribers. So you have to literally buy more equipment to cover more area, and then you make less money because the equipment covers less subscribers on average. Upgrading the equipment then also becomes significantly more expensive.
One is extremely expensive, time-consuming and difficult to cover with high speed broadband, the other is not.