Question

why is hydrogen peroxide kept in glass vessels..?

Answer 1

If hydrogen peroxide is heated or is exposed to trace amounts of metal or metal ions it turns into water or oxygen. Glass bottles sometimes have small amounts of alkali metal ions dissolved in it. This is why plastic bottles or glass bottles coated with wax are used instead.
: If Hydrogen peroxide is not stored in plain (colorless) bottles then light will pass through in and decomposes Hydrogen peroxide into water and oxygen.