What is the reason behind hydrogen's line emission spectrum?
The emission spectrum lines are the changes in the quantum energy levels of the single electron in the Hydrogen atom.
Electrons move around the hydrogen atom in electron waves patterns. These waves occur in distinct quantum energy levels.
The change from one quantum energy level to another energy level has a definite energy level change. There is a wave length associated with each change in energy levels. These wave lengths have a color associated with them.
Low energy level changes have long wave lengths associated with them giving red and orange colors.
High energy level changes have short wave lengths associated with them giving blue and violet colors.
Note each atom has a unique emission spectrum associated with the energy levels and electron structure of the atom. Helium was first discovered by looking at its emission spectrum as seen in the sun.