So this isn't really a history question, but let's see if we can put this in a historical context.

People have known for a very long time that the meat from very young animals was much more tender than meat from Adult animals. Recipes involving the meat from a young calf were present in ancient Rome. Recipes involving suckling pigs were common in both pre-modern China and pre-modern Europe.

It's also important to recognize the source of veal. Veal meat is directly connected to the dairy industry. Female cows only produce milk for some period of time when they are pregnant and then have a calf. If you maintain a large herd of dairy cows, you will be naturally producing calves from those dairy cows and half of these calves are male. The bulk of veal comes from these calves, who, in some cases are fed formula rather than milk (because the milk is the valuable commodity) and then slaughtered at 16-18 weeks of age before they are weaned and are about 450 lbs. A small percentage of veal comes from much younger calves, about 3 weeks old or 150 lbs. Compare this to a standard beef cow which are usually slaughtered between 18 and 24 months of age (and almost always less than 36 months).

I was unable to find any source that conclusively demonstrates when opposition to veal first appeared, but it appears to be a phenomenon of the animal rights movement in the 1980's when it became popular. The animal rights movement itself largely appeared in the late 1970's and early 1980's. PETA was founded in 1980. The first rules regarding modern ideas of "humane slaughter" (requiring animals be rendered unconscious prior to slaughter) were put into place in the mid 1970's. The European Convention for the protection of Farming Animals was passed in 1976.

This web page from UCLA tracks the history of Veal noting that in 1944 veal Consumption in the US was at an all time high at 8.6 lbs per year. the first bill in the US to regulate veal specifically came in 1999, the EU banned using "Crates" to raise Veal in 2006, and the American Veal Association did the same in 2007. Veal Consumption reached an all time low of about .3 lbs per year on average in 2008.

That same source indicates that animal rights activists in the 1980's "heavily protested" the raising of Calves for veal. They have a chart indicating between about 1985 and 1987, Veal was removed from more than 50% of the menus where it had appeared previously.

To put that in context here's statistical data from 1960 to the present

In 1960 the average person consumed 63 lbs of beef, 59 lbs of pork, 28 lbs of chicken (most of which was from meat chickens); and 10 lbs of fish.

In 2019 the average person in the US consumed 58 lbs of beef, 52 lbs of pork, 96 lbs of chicken, and 16 lbs of fish.

That's the "historical analysis" - as i said, your requestion isn't so much of a history question as it is a sociology question. Why was there a push back against veal and not against lamb.

First, I think an important element is the perception of cruelty in the methods used to raise veal (the crates referenced above that were phased out in 2006/7). Second, is a popular perception about the age of a veal calf. An important distinction with Lamb is that a lamb taken to slaughter is developmentally older than a veal calf. A 4 month old veal calf is just at the stage where they are not quite weaned. A lamb is born, weaned at about 60 days (2 months) and lambs are taken for slaughter at between 6 and 8 months of age, typically about 140 lbs. If you see "lambs," they do not appear to be babies. They appear to be young sheep. Veal calves do very much have "infant" traits at the time they are slaughtered. That likely never provided the same emotional impetus to the animal rights movement that existed when you talk about confining still nursing beef-calves into crates so they grow bigger.