The title speaks for itself; from 1440 to 1806, with a brief interruption from 1742 to 1745, the Habsburgs held the office of Holy Roman Emperor. Though it was an elective monarchy, this made it a de facto hereditary monarchy like most monarchies at the time. So why specifically were the Habsburgs so consistently elected to the office?
I thought it was because the emperor could apoint together with the members of the Golden Bull the new king of the romans (predecessor of the emperor). The members of the Golden Bull also favoured a Habsburgian king because it was a reliable decission, it meant security for the members own little states in the big Holy Roman Empire. Also like we saw in Charles V election (his grandfather Maximillian of Austria died before he could appoint his grandson as the king of romans, so also the french king participated in the new elections) money was important. Charles V bought his votes with 850000 gold coins (equivalent of 6'000'000 incomes of a regular worker). So it came down to being close to the former emperor, promising the members of the Golden Bull securtiy and the most important one: money.
source R Vermeir: "Een inleiding tot de geschiedenis van de vroegmoderne tijd".