The can is aluminium, and is connected via the electrolyte filling of the capacitor to some voltage which roughly relates to the voltage on the capacitor. Depending on the design and which foil is the outer layer on the surface, this will tend to be the negative terminal, but leakage current will raise this voltage somewhat. The case is also an unwanted connection, unless it is one terminal or is firmly bonded to the one, typically negative. As an unwanted connection it also form a very small value capacitor, not a very good one, but also one that is very leaky and thus it is never intended to be used.
Thus the case being isolated with the sleeve, and on high voltage capacitors also by robust insulation, and on types with a fixing stud as part of the case the mounting hardware either is designed to provide robust isolation but having plastic parts that need to be installed, or the case is meant to be connected to the chassis ground direct, and is also internally connected to the negative terminal by a tab.