Because the act of driving drunk is, itself, illegal. Drunk driving isn’t framed as vehicular manslaughter of a non-person. If a person is hurt, that’s a separate offense.
There’s a difference between the potential to hurt an unspecified individual, and the potential to hurt a specific entity that may or may not exist in the future.
What was being discussed was not “drinking while pregnant”, but “engaging in risky behavior while intending to carry to term”. Closer to “drinking while intending to drive”.
In any case, I’m still not sure I’m in favor of it. One can’t unknowingly get drunk and drive a car, but one can unknowingly become pregnant and have a drink.