Nothing prioritized Phone 2a over Phone 2 because the Phone 2a is one of its newest devices and best-selling phones. This gives the software team a larger sample size for collecting feedback before moving forward to other devices.
Further, much like other Android OEMs, Nothing relies on chipmakers Qualcomm and MediaTek to provide a platform update optimized for specific chipsets. Nothing says that it receives significant support from MediaTek in this process, which has allowed it to move more quickly on devices with a MediaTek SoC. Instead of holding back the beta program for Qualcomm to provide the update for the Phone 2's SoC, Nothing has gone ahead with the MediaTek-toting Phone 2a as the first device in the Open Beta program.