Because the WTS feels it is difficult to prove gluttony because being fat is not enough; thin people can be gluttons as well. It is like trying to df someone for slander, hard to prove per WTS guidelines.
Many jws here were born after 1973 and were not adults when the use of tobacco, coca leaves, and betel nuts were made a dfing offense.
It was not a health reason but the WTS said it is a form of spiritism.
15. Even though tobacco is not specifically referred to in the Bible, how do Bible principles provide an answer to this issue?
15 The Scriptural evidence points to the conclusion that they do not. As has been explained in other issues of this magazine, the Greek word phar·ma·kiʹa used by Bible writers and translated “practice of spiritism” or “spiritistic practices” has the initial meaning of “druggery.” (Gal. 5:20; Rev. 9:21) The term came to refer to spiritistic practices because of the close connection between the use of drugs and spiritism. Tobacco was also used initially by the American Indians in this way. It can properly be placed, therefore, in the category of addictive drugs like those that provided the source for the Greek term phar·ma·kiʹa. The nicotine in tobacco does not have the same mental and emotional effects produced by “hard” drugs such as heroin or the so-called psychedelic drugs like LSD; yet nicotine addiction does definitely affect the mind and exercises a strong enslavement...
34-36. (a) Why is it reasonable that such practices should now be receiving judicial attention? (b) What responsibility thus rests on those serving as shepherds among God’s flock?
34 It is little wonder, then, that, in these days of widespread addiction to drugs and the growing use of tobacco, those indulging in such things should come under judicial observation. Jehovah God, the Supreme Judge, is at his spiritual temple and is specially scrutinizing those who profess to worship him in that holy place. He has promised to be a speedy witness against the sorcerers or the practicers of spiritism, which from ancient times onward had a connection with habit-forming, enslaving drugs.
35 Do we want to have Jehovah God be a speedy witness against us as addicts to drugs or other habit-forming injurious weeds, things that expose us to the influence of the spirit demons? Jehovah’s judgment against such addicts during the oncoming “great tribulation” will mean their destruction. (Rev. 21:8) Most assuredly, Jehovah God does not want such addicts among the congregation of his Christian witnesses now in this “conclusion of the system of things.” Of the promised “New Jerusalem,” Revelation 22:15says: “Outside are the dogs and those who practice spiritism [the druggers, Kingdom Interlinear translation] and the fornicators and the murderers and the idolaters and everyone liking and carrying on a lie.”
A more comprehensive article on how using the above 3 things, tobacco (not just smoking but chewing and snuff, coca leaves, betel nuts)
WTS comments on gluttony
Actually, whether a person is thin or fat, his attitude toward food has much to do with whether he is being gluttonous. Is food the big thing in his life? If in the presence of others, does he selfishly ignore their needs and take far more than his share? Is he grossly overweight but exercises no restraint at all as to food, habitually gorging himself? Does he feel uncomfortable, perhaps even getting sick, because of having eaten too much? If this is the case as a matter of course, the individual has a definite problem. He needs to learn self-control.