64 QAM versus lower order modulation
There is a trade-off between modulation order, required energy and bit-error rate (BER). As you increase the modulation order, you need to increase the average energy per bit to keep the same BER.

As an easy example, consider BPSK vs 4-PAM. Say you have BPSK with distance between constellation points $d_{min}=1$; this works out to an average bit energy of 0.25 joules per bit.

Now, you want to double the bit rate by going to 4-PAM, but you want to keep roughly the same BER, which means having the same minimum distance $d_{min}$ between constellation neighbors. Now, your average symbol energy is $$\overline{E_s}=\frac{2\cdot1.5^2+2\cdot0.5^2}{4}=1.25$$ joules, or an average bit energy $\overline{E_b}=0.625$.

As you increase the modulation order, you'll see the energy required increase exponentially. Exactly the same thing happens when using QAM.