Speaking only from experience as a third party observer to my Indian friends & colleagues eating habits: onions are often lumped into the same category of 'unpure' ingredients along with garlic. Hindu tradition holds that such things cannot be offered to Gods, and negatively impact spiritual activities. So a non-trivial number of Hindus avoid these foods. Similar lines of thinking apply to other root vegetables. Jain dietary practices also tend to avoid garlic, onion, potoato, and other root veggies for reasons that I definitely do not fully understand.

Aversion to tomato is not something I have encountered though. I suppose it's possible I just haven't noticed it.