Because historically, hardware vendors have had pretty bad security, and are usually not getting reviewed. There's also many of them, which makes any specific one less likely to undergo review.
Meanwhile, BitLocker has received at least some level of review, it is the most common disk encryption product for Windows, and Microsoft can be reasonably expected, based on past experience, to put somewhat competent people on it.
Additionally, at least for parts of BitLocker, there is at least high-level documentation how it is supposed to behave (e.g. https://docs.microsoft.com/en-us/windows/security/informatio..., there may be more detailed documentation elsewhere), plus there is likely reverse-engineered research available confirming the basic functionality.
Meanwhile, BitLocker has received at least some level of review, it is the most common disk encryption product for Windows, and Microsoft can be reasonably expected, based on past experience, to put somewhat competent people on it.
Additionally, at least for parts of BitLocker, there is at least high-level documentation how it is supposed to behave (e.g. https://docs.microsoft.com/en-us/windows/security/informatio..., there may be more detailed documentation elsewhere), plus there is likely reverse-engineered research available confirming the basic functionality.