Part of the beauty about a BBS was that you could access it using anything, even an old or spare computer. All you needed was a modem and text display. An old 286 with a CGA card and 640k RAM was the perfect machine for a BBS, and with an EGA card you could dial in to the few RIP boards that existed.
Sure, you could do so much more with an HTML/browser, but you needed megabytes of ram, a high-res graphics card and monitor, a mouse, and OS like Windows or Mac, a fast CPU...
While it's certainly true BBS access had lower requirements than graphical web browsers, by the time graphical browsers appeared there was a huge population of home systems that could run them.
By 1994 Windows was installed by default on consumer PCs and you'd be hard pressed to buy a new PC that wasn't multimedia capable. The multimedia trend had been pushed by CD-ROM content since the start of the decade. Within a year of Netscape becoming available Windows 95 was the default OS on new PCs. AOL, CompuServe, and MSN also all existed by 1994 and had graphical clients.
So by the time Netscape was first released a majority of PCs in use were easily capable of running it. The number of just AOL users likely dwarfed the number of BBS users even at their peak popularity. C64 and Apple II die hards might have still been dialing their local BBSes but they weren't a majority and BBSes weren't really offering the ease of use as online services and then the web.
Sure, you could do so much more with an HTML/browser, but you needed megabytes of ram, a high-res graphics card and monitor, a mouse, and OS like Windows or Mac, a fast CPU...