Why is Triple DES not vulnerable to meet in the middle attacks?
The computational complexity of the attack you describe is $2^{112}$, since that's how much work it takes to build the look-up table.

In fact, for standard 2-key 3DES like you describe, an attacker capable of building such a look-up table could just as well store $C = E_{K_1}( D_{K_2}( E_{K_1}( P )))$ instead of just $D_{K_2}( E_{K_1}( P ))$ in the table, allowing them to skip the second half of the meet-in-the-middle attack and just directly look up $(K_1, K_2)$ in the table, given a ciphertext block matching the plaintext $P$.

However, the attack your describe (and variants of it) is precisely the reason why three layers of DES encryption cannot provide more than 112-bit security, even though, naïvely, using 3-key 3DES with $C = E_{K_3}( D_{K_2}( E_{K_1}( P )))$ would seem to offer 168 bits of key-space.