Diazoacetic ester is more stable than diazomethane because of the following reasons:

1. Molecular structure:
- Diazoacetic ester has a larger molecular structure compared to diazomethane.
- The larger molecular structure provides more stability as it allows for more extensive delocalization of electrons, reducing the overall reactivity of the compound.

2. Resonance stabilization:
- The presence of the carbonyl group in diazoacetic ester allows for resonance stabilization.
- The delocalization of electrons between the carbonyl oxygen, the adjacent carbon, and the nitrogen atom provides additional stability to the molecule.
- This resonance stabilization reduces the reactivity of the compound and makes it more stable.

3. Electron-withdrawing effect:
- The ester group in diazoacetic ester acts as an electron-withdrawing group.
- The electron-withdrawing effect of the ester group decreases the electron density on the adjacent carbon, making it less reactive.
- This electron-withdrawing effect contributes to the stability of the compound.

4. Steric hindrance:
- Diazoacetic ester has larger alkyl groups attached to the nitrogen atom compared to diazomethane.
- The presence of bulky alkyl groups creates steric hindrance, which reduces the reactivity of the compound.
- Steric hindrance prevents unwanted reactions and increases the stability of the compound.

5. Solubility:
- Diazoacetic ester is more soluble in common organic solvents compared to diazomethane.
- The higher solubility of diazoacetic ester allows for easier handling and manipulation of the compound, reducing the chances of unwanted reactions.

Overall, the larger molecular structure, resonance stabilization, electron-withdrawing effect, steric hindrance, and increased solubility contribute to the greater stability of diazoacetic ester compared to diazomethane.