It's by now impractical for any serious company contemplating being listed in BTC to forego the significant capital available on MPEx in order to favor a different venue (in fact, the cost of doing so can be calculated on the basis of the volume difference, and is in any case significant) for a variety of reasons, and certainly such an attempt would imply the modification of said company to include "being an exchange", which requires rare and valuable skills (programming notably NOT being one of them) and other human capital. Such an effort necessarily spells a movement away from said corp's core mission, whatever it may be (*).
So what you're saying is that no one should list on the NYSE because the NASQAQ has more volume? I call BS.
Different exchanges have different benefits, trade methods, tools for the issuers, end user accessibility, security features, and other pros and cons. Which exchange is best is a matter of best fit, not a matter of volume. Otherwise there would only ever be one exchange in any given currency.
Different exchanges have different requirements- they also operate in different fashions. It is not always which one is a matter of best fit, but rather which one a stock can list to. Stocks also switch exchanges based on a variety of reasons- such as market cap and the extent of their filings.
Exchanges exist to help provide liquidity. Saying that it is not a matter of volume is rubbish.