Why devotees do not take onions and garlic

[Paramananda das]

 

[Sanjaya dasa:] There's a story about a brahmana's wife. She ate a piece of a cow, and some of it turned into carrot, some of it turned into garlic, some of it turned into onion, and the blood turned into red lentils. Is that story true? 
[Srila Narayana Maharaja:] It is quite true; so we should not eat those things. What Vyasadeva has written is true.   Vyasadeva knows everything past, present, and future. He was able to see what qualities are present in each entity. For example, Bhagavad-gita describes the sattvika, tamasika, rajasika, and nirguna natures of various persons, plants, foodstuffs, activities, knowledge, and so on. 
Vyasadeva was able to see what is favorable for bhakti and what is unfavorable. He saw that the onion is tamasika (in the mode of ignorance). It has all the qualities present in the meat of cows, and the same result obtained from eating cow-flesh occurs from eating onion. One will become lusty and violent by eating it. Vyasadeva has also seen in his meditation that garlic came from the bones of that cow. It is full with bad qualities and puts one in the mode of ignorance. Don't eat it. 

In Gaudiya math many strict devotees also do not eat eggplants and carrots: 

VRNTAKAM JALISAKAM KUSUMBHA SMANTAKAM TATHAPALANDU LASUNAM SUKLAM NIRYASAN CAIVA VARJAYETGRJANAM KINSUKAN CAIVA KUKUNDANCA TATHAIVA CAUDUMBARAM ALAVUN CA JAGDHVA PATATI VAI DVIJAH

(HARI BHAKTI VILASA  8/158, 159 KURMA PURANA)

 "One  should  not eat eggplant, banana leaves, sunflower leaves and asmantaka leaves, onions, garlic.  One should not  eat sour gruel  (a thin watery poriage) or the juice of the tree.  One should also give up turnips and beetroots, carrots,  kinsuka, forest figs, and white pumpkin.  If the twice born persons eat these things, they all become fallen."

When the throat was cut some blood dropped on the ground, and became the red onion and the white onion (garlic) respectively. So both onion and garlic originated from the throat and blood of the demons or asuras, thus their consumption brings us closer to tamo guna (mode of ignorance) which characterizes the nature of the demons and thus is detrimental to bhakti. 
(Kamba Ramayana, Yuddha Kanda and Bhagavata, Astama Skandha. Kamba Ramayana is a Tamil text by poet Kambar

One time the rishi who was about to perform a gomedha sacrifice, his wife was pregnant. She had a very strong desire to eat and she had heard that if, during pregnancy one has a desire to eat and does not fullfill this, then the baby that will be born will always have saliva coming from its mouth. Very strange, she desired strongly to eat meat, thus she decided to keep one piece of meat of the cow's body that was offered in sacrifice. She hid it and was making a plan to eat it very soon. At that time the rishi was finishing the sacrifice and uttered all the mantras for the new young cow to come to life. However when he saw the new cow, he noticed that there was a little part missing from her left side. He went into meditation and realized that his wife had taken away a piece of meat during the sacrifice. Now his wife also understood what happened and quickly threw the meat far away. 
Due to the effect of the mantras uttered by the rishi there was now life in this piece of meat. Then the blood of this meat became red lentils, the bones became garlic and the meat became onions.

 Manu-samhita:

5.5. Garlic, leeks and onions, mushrooms and (all plants), springing from impure (substances), are unfit to be eaten by twice-born men.

 

5.19. A twice-born man who knowingly eats mushrooms, a village-pig, garlic, a village-cock, onions, or leeks, will become an outcast.

 

 

palandu lasunam sigrum alambum grjanam palam 
bhunkte yo vai naro brahman vratam candrayanam caret

(Padma Purana, Brahma Khanda 19.10, spoken by Suta Gosvami)

O sages, one who eats garlic, onions, sigrum (a kind of plant), turnips, bottle gourd and meat, that person should observe a candrayana fast.

 

 

vrntakam jalisakam kusumbha smantakam tatha 
palandu lasunam suklam niryasan caiva varjayet 
grjanam kinsukan caiva kukundanca tathaiva ca 
udumbaram alavun ca jagdhva patati vai dvijah

(Hari Bhakti Vilasa 8.158,159, from Kurma Purana)

One should not eat eggplant, banana leaves, sunflower leaves and asmantaka leaves, onions, garlic. One should not eat sour gruel (a thin watery pouriage) or the juice of the

 tree. One should also give up turnips and beetroots, carrots, kinsuka, forest figs, and white pumpkin. If the twice born persons eat these things, they all become fallen.