Expected vs worst-case communication complexity
The reason is that a lower bound on the worst-case complexity automatically implies a lower bound on the expected complexity, so there is no reason to prove the latter.

To see the implication, observe that every randomized protocol with expected complexity $c$ and error probability $\varepsilon$ can be transformed into a randomized protocol with worst-case complexity $c/\varepsilon$ and error probability $2\varepsilon$: The new protocol simulates the old protocol, but forces the simulation to halt after $c/\varepsilon$ bits have been transmitted. By Markov inequality, a forced halting occurs with probability at most $\varepsilon$, so it adds at most $\varepsilon$ to the original error probability.

Since error probability can be amplified, it follows that every randomized protocol with expected complexity $c$ and constant error probability can be transformed into a randomized protocol with worst-case complexity $O(c)$ and the same error probability. Thus, if we show that there is no randomized protocol with worst-case communication $O(c)$ and constant error probability that computes a function $f$, the same holds for protocols with expected communication $O(c)$.