I worked at Volvo for a while and it was explained to us as well as the Service Departments that it would save some money for everyone; owners, dealers, etc. The sensor creates the need to have additional wheel weights installed from the factory, on top of the additional cost of the sensors. This saves money, if even only a few pennies on each car.
TPMS are also notorious for only lasting a few years. I had a car where, like clock work, the batteries would die in the sensors every two years. Decreased sidewalls increase road vibrations and impacts to the rims, which translates into more vibration and internal damage to the sensors, thus damaging the cells of the TPMS Batteries. Being that the trend is to have as big of rims and thin of tires as possible, even across Volvo's lineup, you can see where this would get frustrating for owners.
Lastly, it was determined it was better overall from a safety standpoint to utilize the ABS system and sensors to read tire pressure as opposed to going off of actual psi readings without the use of the TPMS sensors. First off, being that typically replacing a failed TPMS sensor is about $125 or more, studies have shown that owners would put off repairing/replacing failed sensors and *ignore* the constant warning light. If one or more tires were low or blew out, an owner would not know until it was too late, as the light was already on and they were accustomed to already seeing it illuminated. Some would even remove them altogether and have the system deactivated. GM dealers constantly do this when selling used cars instead of replacing the sensors as it saves them some cash and labor costs since no one will ever notice the difference. Utilizing the system implemented for the TPM system in our Volvo's guarantee uninterrupted functionality of the system for life, and, also guarantee that an owner will always know if a tire is low, blows out, etc, and can never be unaware. Apparently, though I don't know how true, this system is more accurate from a safety standpoint than the sensors which read based on actual psi. Even though you cant see the current pressures within the system, you know it is within acceptable range if green.
Since the system doesn't function off psi readings alone, you also will never get the "low tire pressure" warnings just from temperature swings. Other cars I had would always illuminate the light in the early mornings, and shut it off once it warmed up outside!