Depending on how you count, the modern extended range bass has been around since the mid 1970's. That is to say, it is almost old enough to be a grandparent. Why is it still seen as exotic?
I noticed today that some of the traffic to this blog (according to Google Analytics) comes from people Googling "7 string bass." On one hand, I find this flattering, based on my shallow understanding of Google's PageRank system. The other hand, as you have probably deduced, is more problematic.
After all these years why would my blog, which isn't even specifically about bass, be one of the more relavent sources of information about 7 string or other extended range basses? Seriously, my instruments that I have written about here aren't even really basses.
The reason is that extended range instruments, be they >6 string guitars or > 5 string basses still aren't mainstream. Have you noticed that Sweetwater doesn't even carry new 7 string guitars? They have 40+ solid body Ibanez models on their web site (as of today) but the sole 7 string is in their used gallery. (Although they do currently carry the Ibanez RG2228 8 string, which I find odd.)
I like to talk about moving beyond convention and creating unique things, but when you think about it extending the range of a stringed instrument isn't exactly a revolutionary idea. Once upon a time banjos all had four strings, but now 5 strings are standard, with the additional one not even being the same length as the others. Why have multi-string guitars and basses not penetrated deeper into the mainstream?
That's a rhetorical question. It's inertia. It's convention. It's the human tendency to think inside the box and stick with what's familiar. It's also a sad commentary on people's ability/willingness to grasp and accept a fairly simple idea that can expand their musical horizons.