Only spells that target either a creature or an area are eligible to be stored:
Spell Glyph. You can store a prepared spell of 3rd level or lower in the glyph by casting it as part of creating the glyph. The spell must target a single creature or an area. ...
Glyph of Warding is a Touch spell that targets a surface or an object that can be closed. Eligible spells must target a single creature (like eg. Witch Bolt) or an area (like eg. Fireball).
While Glyph of Warding does cover an area in a sense, it does not target an area. There are three important locations for the spell:
- Where the spell was cast (unimportant otherwise, but the glyph's undone if it is moved far from its original site)
- Where the glyph is (actually determines when the glyph is triggered)
- Where the creature triggering the glyph is (determines where the resulting spell is cast)
Seeing that the Glyph can be moved from its original location, my interpretation is that the target of "Touch" means the spell's target is the object or surface that has the glyph. However, the object or surface can move and the glyph moves with it (functioning only up to a small distance, but still!) suggesting that indeed the spell doesn't target the abstract area, but the concrete object, be that wall, door, book or floor.