Because of politics. Because firstly, killing a cow is not like killing a dog: in most cases, cows are killed instantly whilst when dogs are killed by sadistic bastards, they're tortured, burnt, hurled around. The vicious motive behind it.

Secondly, why they do it. Because most of the cases of people screaming at cows being killed end that the victims were poor; because a poor person eating a cow out of neccessity is nowhere near someone torturing and killing a dog for fun. Personally, if someone was starving and they ate a dog, I would not complain, for who am I to judge?

Thirdly and possibly most importantly, politics. We know who the cow-killing laws are targeting. And we know how with these laws, certain groups of people, who haven't ever killed a cow but only transport bodies/skin cows have been brutally abused. When it comes to dogs - immediate action is taken usually, as with that kid who threw the dog off the balcony. Cow laws are far more damaging than dog laws; to beat up and humiliate people in public for carrying cow skin when they do not find it wrong to eat a cow! Is that not like me thinking you eating lentils is wrong and beating you in public for it? Or even more similar, us going to China, disgusted by them eating dog meat - and beating up all the Chinese people.

I'm not saying cows should be killed and dogs should be saved, or vice versa. I'm saying you can't compare these, at least not in India because both the personal and political motives are very different.