Animals: why did the Buddha remain silent?

At the time of the buddha, most lay people (ascetics or householders) were not buddhists at all. There were plenty of other teachings with their rituals and believes to follow. The buddha was not interested in changing all lay people's life and less so managing them, especially for the lay people who are not even buddhist and just follow other religions (so they would give food to bikkhus because they either like it or because it is good in their religion). Since the buddha manages the bikkhus and ''a monk lives depending on a village or market town", it is up to the monks to accept or reject things given to them. It is in the other religions that puthujjanas have created, where there are rules for lay people, and monks. And with the secular people, there is no monks at all, so they create rules just for lay people.

For the buddha killing animals in offerings made for him is never meritorious like he says in mn.055, and the other parts of their rituals, like mantras, chanting, dancing, burning fire and so on like in mn92 , are useless for merit and for being enlightened, so he just changed the killing part, which the brahmins viewed has meritorious like the ''great sacrifice which was being prepared for brahman Uggatasarīra" in an7.44, when those lay people wanted to make merit with the buddha... In the vinaya, and many more in mn.051, there are also 5 things that puthujjanas see as meritorious, but are in fact unmeritorious, and the bikkhus ''abstain'' from those

Five unmeritorious gifts are considered by the world to be meritorious 4 : a gift of intoxicants, a gift for a festival, a gift of women, a gift of bulls, a gift of pictures.

For The lay people who like to feel buddhist, they would follow at least the 5 precepts and Killing animals is covered in that. In fact the 5 precepts is for anybody who wants a good birth and good present life like the buddha says to the brahmins and the population managed by the brahmins http://www.palikanon.com/english/pali_names/vy/veludvaareyya_sutta.htm

The bikkhus do not control what food is put in their bowls, that's part of the whole ''being content'' with whatever they have, what is given to them, no choosing the people who give them stuff (unlike some bikkhus who tried to get nice food from a lay person and when some bikkhu denied this (the famous teenager arahant), they tried to get him excluded with the help from a bikkhuni) , or what they find in trashcans (like their robes). but the bikkhus can choose to eat whatever they want inside their bowls, then they can throw what they dislike or the leftover, but in some special places.

This is mere speculations from me, but turning down meat was probably not a viable option during buddhas lifetime, for the sake of survival. A layperson would likely run the risk of starvation without meat, and making vegetarianism a decree could possibly put a lot of peoples health at jeopardy.

Again, i am guessing now, but my point is that we can probably only understand a set of beliefs by understanding the context of that time. It could explain the "why" of buddhist ethics. (We could for instance compare this to the muslim decree on abstaining from eating pork, which at least partially is assumed to be for health reasons to my understanding).

You also seem to ask what ethics regarding vegetarianism should be in order to be consistent, which is a bit different than the question why.

Your argument regarding today's meat industry seems anachronistic, since the premises of meat production in those times was presumably a lot different than the large scale meat industrialization of our times.

In summary, the choice of eating meat is likely more feasible today, and therefore the ethical aspects of that choice becomes more salient, compared to buddhas time. Back then, it's possible that the ethical conundrum could be sufficiently solved by deriving karma from our intentions.

Applying this ethic in todays world, with the meat industry that goes with it may make a much stronger and convincing case for vegetarianism, compared to buddhas time.