> why can't we start using more concurrent TCP connections with HTTP 2 when it makes sense
Because using 6 TCP connections per site is a hack to have larger initial congestion windows, i.e. faster page loading, ending up using more bandwidth in retransmission instead of in goodput. Instead we could have more intelligent congestion control algorithms in one TCP connection to properly fill up the available bandwidth. See https://web.archive.org/web/20131113155029/https://insoucian... for a more detailed account (esp. the figure of "Etsy’s sharding causes so much congestion").
Because using 6 TCP connections per site is a hack to have larger initial congestion windows, i.e. faster page loading, ending up using more bandwidth in retransmission instead of in goodput. Instead we could have more intelligent congestion control algorithms in one TCP connection to properly fill up the available bandwidth. See https://web.archive.org/web/20131113155029/https://insoucian... for a more detailed account (esp. the figure of "Etsy’s sharding causes so much congestion").