They likely didn't have a choice. In order to stream fast enough from the card it requires the SD card to have direct access to the PCIe bus. The XBox and the PlayStation 5 both require PCIe based mass storage solutions. The XBox X and XBox S both require expansion cards to contain SSD's using PCIe 4.0 x2 lanes. The storage expansion for the Playstation is also an PCIe 4.0 NVMe SSD. The Switch 2 requiring a single PCIe 3.1 lane is not surprising.
It can be easily seen why they need to move to this new standard. The UHS-III standard that the Switch used was limited to 312 MB/s full duplex (which means data going in both directions at once). The new SD Express standard allows for 985 MB/s full duplex, and that is at its lowest speed. At it's highest speed it can reach 3,940 MB/s utilizing 4.0 ×2 PCI-Express lanes, the same requirements that the XBox X and the Playstation 5 use.
Plus, it's not like this is a new standard or anything. It has existed since SD standard 7.0 which was released in June 2018. The current version of the standard is 9.1 released in October 2023. It's just that no one has really had a major need for it, except in normal SD sizes for prosumer video cameras. The Switch 2 is the first major device to need the Micro SD variety of SD Express. Unfortunately that will mean limited supply to begin with, and likely higher prices, but I have no doubt it is necessary.
I saw we should be glad that Nintendo is embracing recent standards since we won't likely see another one for 7-8 years such as with the original Switch.