Why does botulinum toxin seem to be more dangerous to humans than to other mammals?
Because of that, I assume botulinum toxin is more dangerous to humans than many animals.


Couldn't find too many examples but, there are some things to consider (according to [1]):


there are seven distinct types of toxin with variable action among animals 
different dose / effect intensity ratio between toxin types
toxins A, B, E and F cause disease in humans
toxin C and D are more common to birds, cattle, horses


And here are some numbers:


cattle median lethal dose: 0.388 ng /kg [2] of C type toxin
mouse median lethal dose ( 12.88 times more than cattle): 5 ng/kg [2] of C type toxin
human median lethal dose: 1 ng/kg [1] of A type toxin


When it comes to carcasses toxicity, there are some papers to consider:


Smith GR, Turner A. Factors affecting the toxicity of rotting carcasses containing Clostridium botulinum type C. Epidemiol. Infect. 1987 Jun;98(3):345-51. PubMed PMID: 3297745. - Persistent toxicity at 20 and 37 degrees Celsius even for a long time. No toxicity at 16 degrees Celsius
Smith GR, Turner A, Till D. Factors affecting the toxicity of rotting carcasses containing Clostridium botulinum type E. Epidemiol. Infect. 1988 Jun;100(3):399-405. PubMed PMID: 3288491. - Maximal toxicity at 30 degrees Celsius that is lost in time.



  are they better in avoiding harmful substances based on more suited olfactory sense?


Not if we are talking about nonproteolytic B, E and F type bacteria. Indeed, Clostridium botulinum, proteolytic type A, B and F types produce odor [3].


  Or are we just more afraid?


I don't see how fear could (significantly) influence pharmacodynamics and pharmacokinetics of the toxin.



References:


CIDRAP - Center for Infectious Disease Research and Policy. Botulism. Available from http://www.cidrap.umn.edu/infectious-disease-topics/botulism
Moeller RB, Puschner B, Walker RL, Rocke T, Galey FD, Cullor JS, Ardans AA. Determination of the median toxic dose of type C botulinum toxin in lactating dairy cows. J. Vet. Diagn. Invest. 2003 Nov;15(6):523-6. PubMed PMID: 14667014. 
P. Kendall. Botulism. Colorado State University Extension. Available from http://www.ext.colostate.edu/pubs/foodnut/09305.html