Most importantly, they differ in standardization and the use of spectrum: NB-IoT is a 3GPP industry standard using existing assets of network operators like sites, base stations, antennas, backhaul and licensed spectrum. Licensed bands can handle a massive number of devices, stable and securely at high transmission capacities and with high reliability.
Sigfox and LoRa are proprietary technologies on unlicensed bands and therefore need dedicated network components. With an increasing number of connected devices, data transmission on unlicensed bands is likely to deteriorate (e.g. by interference) while benefits of using a standardized technology in licensed bands will become even more important.