"usually" and "typically" are doing a lot of heavy lifting here :)

Access to knowledge, equipment, and budget varied dramatically prior to widespread internet access. Someone setting up a BBS might not even know about multi-line modem cards or serial port expansions. Even if they knew about them they may not have been able to reasonably obtain them. Or they may have been operating on donations, surplus, or discount equipment. Or they simply may not have had the luxury of time to research all of that as user demand meant they were too busy laying tracks in front of the train.

Many BBSes ran on 1-2 lines per PC because that's what they understood how to build or the hardware they had access to. You might be surprised at just how many lines some BBSes setup this way had!

People forget there was a time that anything outside the standard PC was extremely expensive, often had flaky or nonexistent software support, locked you into a fly-by-night vendor that might go out of business tomorrow, was only available via a distributor who wanted to have you talk to a "sales consultant" before they'd sell you something, etc. Many many people chose sub-optimal implementations because it was an off-the-shelf PC they could replace at any time with trivially simple software requiring no special CONFIG.SYS drivers or TSRs to fiddle with. Especially if you'd ever been burned previously.