Because it's fast enough you don't notice it's "slow" outside of benchmarks?
Theoretically 802.11a is the same speed as 802.11g, but in practice it drops off super quickly and means a “simple” page (like gmail/outlook) is loading in 30s-1m.
Not sure if that was a factor or just that being less popular there was less investment to make it "better".
I don't think I ever saw a card supporting 802.11a exclusively, it was part of a chip supporting multiple standards. And then you would always use 802.11g.
This ignores the historical fact that 802.11a cards predate 802.11g by multiple years - A-only cards were available in 2001 when the initial draft of the G standard was announced.
802.11g was still a draft. There were serious issues to use cards from different vendors to talk to each other, and 802.11b cards were common.
Anyway, 802.11a frequency didn't play well with walls (or other solids), added to the shorter range compared with 802.11b (and g), in 2003 b was king and g was introduced early because of the higher speed.
Nope, that never happened to me. I use 802.11a for the same reason as OP (Intel card on FreeBSD plus a 5GHz only network), an iperf3 test reports a perfectly stable 22.2 mbit/s. More than enough for surfing, in fact Tor limits the bandwidth more than this Wi-Fi :)
I tried it again more recently, and was very pleasantly surprised - it's not as fast at the web, but at least for web pages (as opposed to downloading files) the difference was much less noticeable.