A few immediate thoughts: Transaction volume is ultimately not the important metric--revenue is. And, following [1], it seems that the total revenue for HFT was probably around $2Billion in 2013--for a whole industry, that's not very much! Measuring transaction volume is akin to comparing shipping between Amazon and Walmart ignoring the fact that Amazon ships directly to consumers while Walmart mostly ships to large…
Surely the 15+ hour shut downs are a much bigger limit to liquidity than a few microseconds here and there?
There's obviously no technical reason - I don't see Amazon or Google closing down their websites from 4pm to 9:30am. And if it's about the release of news, that only really needs a window of an hour or so.
[1] https://en.wikipedia.org/wiki/List_of_stock_exchange_opening...