I recognize BTTTX as one of the old Benham target maturity zero funds.
Why wouldn't you want it? I would say it was because you would be chasing past returns in a market sector that's more richly valued than ever in my lifetime. It would be no different than asking why not buy stocks in early 2000 -- after all, the last 18 years had been terrific, but there is this thing called extreme overvaluation to worry about. And it would have been pretty absurd to expect the 2000-2018 period to be anything like 1982-2000 based on relative stock valuations in 1982 and 2000.
It would be because bonds in general (and zero coupons in particular) have had a great run because of a long-term trend in declining long-term interest rates that began around 1984 and continues to this day with a couple of short interruptions.
If you think bond interest rates have much more room to go down, this fund should continue to do well. And though my crystal ball is murky, I just don't see how they can go too much lower. It seems to me that there is relatively little upside potential compared to downside risk for zero coupon bonds. And zeroes are far more volatile and interest-rate sensitive than regular bonds that pay interest; when rates fall zero funds will rise much more sharply than regular bond funds, and when rates rise they will plummet much faster.
"Hey, for every ten dollars, that's another hour that I have to be in the work place. That's an hour of my life. And my life is a very finite thing. I have only 'x' number of hours left before I'm dead. So how do I want to use these hours of my life? Do I want to use them just spending it on more crap and more stuff, or do I want to start getting a handle on it and using my life more intelligently?" -- Joe Dominguez (1938 - 1997)