Many great choppers have been built on Sportster frames. When you're interested in creating a hardtail chopper there's no more popular place to start than with a Sportster. The financial reasons are obvious with it being the lowest price bike in the Harley Davidson line up, but that's not always the reason that guys build their bikes around the model.
Some bikers just want to keep it simple. Using a Sportster frame that's been modified, or a completely custom Sportster frame allows the builder to keep the bike light and still have tons of relative horsepower. This makes for faster and easier handling custom bikes.
Sportster chopper frames also allow you to throw in a more powerful and in most cases, better tuned engine that you've salvaged from a Buell. Any additional components that you need are readily available and entire choppers can be built for a very low cost while still maintaining quality and handling.
If you're looking to make a rigid on the cheap many weld on hardtails are available that will allow you to keep that most important part of the frame for the DMV: the stamped in VIN. Assuming you're not also going to stretch the neck.
How many of the bikes that we first referred to as "choppers" were built on Ironhead Sportsters? More than few, that's for sure. These were bikes guys loved to chop.
And I can think of one last reason to use the beloved Sportster frame as the starting point of your rigid frame project: if you're going to end up with a custom chopper that has no suspension, you might as well start with a stock bike that pretty much has no suspension to begin with.
