Cardiorespiratory fitness is dependent upon both the endemic level of exertion required for an activity and the unique physiological and postural elements of that activity. MVO2, and other metrics of fitness, are partially determined by genetics, then developed through exercise, and are not the focus of this comparison. Please understand that the standard distance for rowing competitions is 2000 meters (although there are other distances used), so we are discussing the physiological changes incurred while participating in an event lasting approximately 6 minutes. Almost all skiing and biking (except velodrome sprints) events are conducted over far longer distances. It's beyond the scope of this discussion to examine the three energy systems used in these events, suffice it to say that both anaerobic and aerobic components are present, and used in ways unique to the event.
The main reason for the strengthening and enlargement of rower's hearts lies in the unique physical postures of the sport. The heart is the center of a double-circuit plumbing system. The right side of the heart (atrium/ventricle) receives deoxygenated blood from the venous system through the inferior and superior vena cava. This blood is pumped through the lungs via the pulmonary artery, where it enters all the minute branches that allow exchange of oxygen and carbon dioxide. The "freshened" blood then leaves the pulmonary circulation through the pulmonary veins and enters the left side of the heart (atrium/ventricle).
Before we continue, let's look at the factors that influence performance of the right heart. If we assume a heart is capable of a certain "output", just like the pump in your backyard waterfall pond, then what factors can change how much liquid is pumped? First, the pump must receive an adequate supply of volume in order to reach its rated output. Second, the downstream resistance must be low enough to prevent "back flow" pressurization effects. Third, since the heart works like an expanding and contracting balloon, it must be free of external compression that would restrict its excursion. There are other factors that may influence pump performance (viscosity, temperature, etc., but these may be ignored since those parameters will be very similar among athletes in different sports).
The left heart performance is affected by similar factors. The amount of blood available to pump, the amount of external compression applied, and the resistance to outflow all influence the final result. The resistance to flow is higher for the left heart, since blood must be pumped all the way through the body into progressively-smaller diameter vessels as the periphery is approached. The aorta and aortic arch are quite elastic, as are the smaller arterial vessels to lesser degrees as you get further from the heart. This elasticity is called a "windkessel" function, meaning that the aorta receives the high pulse pressure from the left ventricle, expands to accommodate the instantaneous rush of blood, then contracts/rebounds to its original size as the blood flows away. This effect causes the pulse pressure (difference between systolic and diastolic pressures) to decrease as you get further from the central blood vessels, almost attaining a pulseless state in the smallest capillaries. This windkessel function is affected during rowing.
Rowing, due to the unique bilateral/symmetrical motion pattern, creates a horrid situation for the efficient operation of the heart. All three of the factors I cite above are negatively affected during rowing. The "catch" position, the coiled posture assumed just before one pulls on the oars, draws BOTH knees up toward the chest simultaneously. Running, riding, cross-country skiing, and countless other sports use an alternating bipedal pattern. This "crunch" produces high intra-thoracic pressures. Since venous blood return to the heart is almost completely passive, higher intrathoracic pressure acts as resistance to that blood flow, thereby limiting the amount of venous blood available to the right heart. High intrathoracic pressure also acts to "compress" the heart, forcing it to work harder. Finally, increased intrathoracic pressure also limits the expansion of the aorta and diameter of the downstream vessels, thereby creating a higher resistance to outflow of blood from the left heart. The bilateral thoracic compression also limits the excursion of the diaphragm, increasing the labor of breathing.
All of these factors combine to produce a heart that becomes larger and stronger. This has nothing to do with athletic ability, etc. It follows the SAID (specific adaptation to imposed demand) principle. As to lateral movement and many other athletic qualities, I absolutely agree. In fact, the reduction of the senescent period has been linked to the maintenance of muscle mass, putting weight training as an important component of true fitness. All one has to do is compare a marathon runner to a ¼ mile runner to understand the physiological effects of high-strain exercise. These short rowing races are extremely strenuous, in a manner that could not be sustained for typical durations of biking or cross-country skiing events.
Lee