The percentage of people who are competent enough (or crazy enough) to know a LISP well enough to build a product out of it AND be interested in a startup AND applying to YC is quite small in comparison to the number of people slinging code in the more popular languages (who are also interested in applying to YC).
Of course YC is funding companies that don't use i.e. LISP/Haskell/whatever is cool. If it only limited itself to the elite/hipster programmer startups (heh), it wouldn't have that many companies to invest in per year.
I know this comment wasn't addressed to me, but that's why my original question was phrased as: Are Lisp startups more successful, not: Are there more Lisp startups?
This was more or less my take. When Viaweb was built in the mid 1990's, Python was not a mainstream language, Java was new, and building web sites in C++ was approximately as efficient as it is today. The scale of the web was different in that Viaweb had a server in their office. It was also the server.
By the first batch of YC, Graham had written about Python as an example of a leading edge language that was a good way to filter for "better programmers". Not long after the first batch started, Reddit had ditched Common Lisp for it.
Anecdote is not the singular of data, but Whatsapp choosing Erlang is a more recent example of using the right tool for the right job.
I think it's fair to say they are based on PG's early experiences, but it didn't appear to generalize.
You'll note he stopped writing about it, and YC funded plenty of companies that used many, many different languages.