Scientific reasons for not eating certain animals?

Muslims are not allowed to eat certain animals like pig, dog etc. but are allowed to eat cow, chicken, lamb etc. I don't understand the reason behind this refraining from eating some animals but are allowed to eat others. **If eating meat is the issue, Allah should have commanded us to remain vegetarians!** Is there any scientific explanation for not eating certain animals? Some say that pigs eat filthy things and the meat is not healthy for consumption. However, if the pig is reared in a clean environment, won't it make it a clean animal?