Why do TCS papers have author names in alphabetical order of their surnames?
The American Mathematical Society has released a statement (pdf link) about the commonly accepted practice of listing authors in alphabetical order in mathematics. Their reasoning applies, in my opinion, to TCS as well.

This kind of theoretical research is often done in small groups of 2-5 people. In close collaboration it can be difficult to determine who came up with which idea and/or whose ideas are more important for the main results. Using the alphabetical order lets us circumvent arguments about it and default to "everyone contributed equally", which is often (but not always) more or less the case anyway. It's also easy to explain to non-mathematicians by e.g. showing the above statement. When you apply to math/TCS positions, the hiring committee will be aware of this convention and won't hold your last name against you. My impression is that researchers in fields close to math/TCS tend to know about it as well.

In biology and other fields where research is based on experiments, papers are written by large groups whose members have distinct roles. A professor has an idea and obtains funding, one researcher designs the experiments, some lab assistant runs them, someone else analyzes the results, others write the paper. In this context it makes more sense to differentiate the authors by their contribution.