Why are left bundle branch blocks worse than right bundle branch blocks?

Two > One. It implies that two of the three "wires" feeding the lower chamber are out instead of just the one that causes a right bundle.
The cause important. The right bundle is relatively easy to damage, so relatively small injuries can create it. The left bundles (two of them, one easy to injure, one hard to damage) require more damage to interfere with their function, so they are associated with "worse" heart disease with the prognosis inherent in your question. Cause is important, so need to talk to your doctors to sort those out.