There are several ways to make a non-alcoholic beer (which BTW is not alcohol free. These beers can have .5% ABV). Boiling off the alcohol at 175f degrees, vacuum distillation at 120f degrees and reverse osmosis filters. They all take a toll on flavor and mouthfeel.
Here is a great article:
The most common way that alcohol is removed from beer is through heating. As we've discussed in several previous articles, alcohol has a much lower boiling point than water. At sea level, it's roughly 173 degrees F.
To minimize this, some operations practice vacuum distilling. Depending on the power of the vacuum, the alcohol's boiling point may be lowered as far as 120 degrees, which is much less disruptive to the flavors.
Then the last way:
"...beer is passed through a filter with pores so small that only alcohol and water (and a few volatile acids) can pass through. The alcohol is distilled out of the alcohol-water mix using conventional distillation methods, and the water and remaining acids are added back into the syrupy mixture of sugars and flavor compounds left on the other side of the filter.
The reason why hoppy and dark beers can't really be made this way:
According to Brew Your Own, "The hop aromas will usually be driven off within the first five minutes, while the hop flavors will be gone within the first 15 minutes." This is why neither we nor any of our sources have encountered a half-decent non-alcoholic IPA, which is a damn shame, really.
To sum it up, light lager type beers have little color or flavor to begin with so it's easier for them to come through the process pretty close to what they were before they went in. Hoppy or dark beers would be stripped of so much flavor and color that they would be awful so nobody tries...