Hydrogen is placed in the main group of the periodic table since it has only one electron in it and it also behaves like alkali metal properties.
Hydrogen atom:
Hydrogen forms a univalent positive ion group 1 elements, but it also forms a univalent negative ion group 17 elements. Also, hydrogen is a gas and forms a molecular diatomic compound just like group 17 elements.
This is not the case with other group 1 elements. We can see that hydrogen is chemically and physically dissimilar to other group 1 elements hence one can logically argue against its inclusion in group 1. Also, it is not a halogen, so we can also argue against its inclusion in group 17.
If we place hydrogen in group 1, we can understand that it forms univalent positive ions and if it is placed in halogen group it will form univalent negative ion.