This is just the default configuration BitLocker used for a long time, there is a no hardware constraint that causes this. However, pins are easier to remember,

This is just the default configuration BitLocker used for a long time, there is a no hardware constraint that causes this. However, pins are easier to remember, usually guaranteed to be available at the pre-boot time the tpm needs unlocking and as the TPM ships with anti-hammering abilities they offer sufficient entropy to protect against attacks.