The real reasons are that 11a is still good enough for many use cases, and that OpenBSD's wireless developer department is seriously understaffed (apply by sending a patch).
Legend:
PHY: think of this as "wifi radio"
HT: "High Throughput" aka 802.11n
"Clause 20": HT PHY specification
"Clause 19": 802.11g PHY specification
"Clause 18": 802.11a PHY specification
"Clause 17": 802.11b PHY specification
STA: any 802.11 capable device
[[[
20.1.1 Introduction to the HT PHY
Clause 20 specifies the PHY entity for a high throughput (HT) orthogonal frequency division multiplexing (OFDM) system.
In addition to the requirements found in Clause 20, an HT STA shall be capable of transmitting and receiving frames that are compliant with the mandatory PHY specifications defined as follows:
— In Clause 18 when the HT STA is operating in a 20 MHz channel width in the 5 GHz band
— In Clause 17 and Clause 19 when the HT STA is operating in a 20 MHz channel width in the 2.4 GHz band
]]]
Translation: All of 11a/b/g are a proper subset of 11n.
11ac is 5GHz only. I assume it's backwards compatible to 11a but I haven't read the 11ac spec.