Essentially your question boils down to why are some sins automatically excommunication (Latae sententiae) and some not. Now, in the US for decades a priest can forgive an abortion. The Bishops made this possible in many places after abortion was legalized. First let's state that there are mitigating circumstances of culpability that could make someone not subject to the penalty. But that is another discussion. Also in danger of death any priest can absolve from these even without permission.
Also let's be clear, a priest can absolve them. But the penitent then must seek the Bishop or an approved confessor within a month to meet and make sure the behavior will not repeat. Referral can be made by the priest.
But really we need to look at why some things are automatic excommunication. There are Nine. And it is a condition that to be subject to the penalty you must know it exists. You must know the act will incur the penalty and do it anyway.
Abortion (Including all who assist with it in Formal Cooperation.)
Apostasy
Heresy
Schism
Violating the sacred species (desecration of the Eucharist)
Physically attacking the pope
Sacramentally absolving an accomplice in a sexual sin
Consecrating a bishop without authorization
Directly violating the seal of confession
Now for Apostasy, Heresy, attacking the Pope and Schism we see sins against the visible Church. So an automatic penalty and extra steps to reconcile are necessary to be sure the issues are worked out. The same for consecrating a Bishop without permission. For the other we see sins directly against the Sacrament of Confession by a priest. This is also a violation of vows and the extra steps are needed to be sure there will be no repeat and issues that caused it are gone. Then we have desecration of the Eucharist. This shows such a turn from what the Church is that an automatic penalty is needed to make sure the underlying problems are solved.
So that leaves abortion. So what makes it a case that is different from other sins like murder and the variations on such...like infanticide. The basic answer is that the Church decided that in the case of the murder of the most innocent at the most vulnerable point this would be the process. She could have decided that it would happen in all cases of murder or illegitimate killing, but chose the point at the height of vulnerability and innocence. And since one must know that the penalty exists for it to be incurred (it is always a sin but I am speaking of the canonical penalty) if someone does it anyway they are basically turning to the Church and saying that they do not care about the authority.
All mortal sins divide us from God. But some of them also are direct oppositions to the Visible Church and our union with Her. The priest also has the ability, if he deems that it is too burdensome for the penitent to remain in sin until the Bishop absolves...he can absolve the sin. That is up to the prudential judgement of the confessor. He can then inform the Bishop but the seal of confession still applies.
Also I will say that it is more common that the canonical conditions make it much harder to the mother to receive the penalty due to issues like lack or knowledge, compulsion, mental state..ect. More often than not it is the doctors and providers who are incurring it.
Remember this is a function of canon law and can change. There used to be more automatic ones before the 1983 code. And the 1990 code for Eastern Catholic Churches has eliminated them all.
But the best answer as to why abortion and not similar sins is that it used to be rare and illegal and was not just a sin but an organized underground activity and this process helped to end people's involvement in it.
I think as we get other codes of canon law in the future there will be less and less automatic penalties except those that are a direct break with the Visible Church.