Why supporting animals ethics is not always an obvious decision.
Your logic relies on an incorrect equivalency of animal suffering and embryonic death.
If you look at the marketing produced by vegan-advocating organizations in the US and Europe (and this is who you're talking about, not apparently, the millions of non-animal eaters in India and south east Asia), the "animal suffering" argument comes down to the suffering livestock animals endure while being raised (in CAFOs, for example). I think if you were to ask vegans, it's not so much about killing animals (many of them would have no problem setting cockroach traps or swatting a mosquito) but about raising animals solely to have them suffer while alive, and die to feed humans.
Anti-abortion rhetoric, on the other hand, relies on an argument about the immorality of ending the potential life of the embryo. All the anti-abortion marketing is about the potential of the life, about how the embryo is a whole (potential) human, about interviews with adults adopted as children who are happy they weren't aborted, etc. I've actually never seen an appeal to end the suffering or pain of the embryo (probably because it would be much more difficult to convince people of).
This is the problem: vegans want to avoid suffering by eliminating the suffering life, while pro-lifers believe the life itself is sacred and should be preserved, regardless of the suffering it causes (for the parent or the child during it's life). These ideologies are just fundamentally focused on optimizing different things, and as such, aren't useful in informing the other.
Forced pregnancy constitutes a massive violation of a person's bodily autonomy, while being prevented from eating meat does not. The violinist argument (https://spot.colorado.edu/~heathwoo/Phil160,Fall02/thomson.htm) is at least historically common in arguments about abortion. If one replaces the violinist in that argument with a cow, even people who think killing or confining cows is wrong, such as vegans, would likely think that forcing a human to have their body used to sustain an animal for 9 months would be more wrong.