"Mack" gave as good a logical explanation of the inconsistency as is likely to develop. A political view would be that politicians don't have the guts to take on abortion directly but still need the support of anti-abortion activists to win elections. They seek out related matters which are less controversial, pass laws on those, and then present the results as progress. In this case, charging the murderer of a pregnant woman with double murder is something that will not be strongly opposed while allowing the politician in question to lobby support from anti-abortion activists.

In other words, the answer to your question is "political cowardice".