One needs to remember that in the United States, the District Courts are under the precedence set by the Appeals Court they are part of. So if there is a certain way things are done, rulings that have not been overturned, or just a general sense of "expertise" based on a lawyer or firm's experience with a certain court, an entity will file a lawsuit in what they think is home turf.
The odd thing here is that unlike other types of litigation, patent lawsuits have their own Appeals Court, which is the Appeals Court for the Federal Circuit. They have established a set of ground rules for jurisdiction, and it seems to include a test that looks for both parties doing business in the jurisdiction. Heartland does not do direct sales, have offices, or even registered in Delaware, but apparently they have a couple of national contracts who ship the "accused products" into Delaware, which according to the court means that jurisdiction of the Delaware court is established. Seems like a bit of a stretch, but those are the rules.
Normally if two Appeals Courts come to different rulings for cases with similar facts (see the same-sex marriage and Obamacare cases), the Supreme Court will step in and adjudicate the matter, thereby setting nationwide precedence. But since there is no other Appeals Court with the jurisdiction to hear patent cases, I'm not sure how that would work. It seems like it would take some pretty strong evidence of misinterpretation or misrepresentation of Congressional intent to force the Supreme Court to step in. The only remedy here is specific legislation to override the precedence.