Because features available on firmware 2.1.7 requires SD card. If you don’t want these features, you can go back to firmware 2.1.6 which does not require SD card.
If it stops booting at ‘Connecting’ message, it doesn’t sound like a SD card problem — it will get stuck at Error Code: 0x2D if SD card is not detected.
Also, since your built the circuit yourself, it’s hard for us to tell if there is any soldering/reflow related issues.