It's easy to understand why, though, apart from the general slow uptake of new brands and phone lines. The 6T goes for $550 at T-Mobile now, while the LG G8 ThinQ, for example, is $620 and the S10e is $750, plus they get launch deals and promos to chop those prices even further.
The slow uptake of the OnePlus 6T bit comes courtesy of Counterpoint research as part of a pitch that investigates how less popular brands like OnePlus or Google's Pixel fare in the real world. decent, apparently, but not great, as the Pixel 3 was 7% of Verizon's sales in the same quarter. Most of the switchers to those two came from entry-level Samsung or Apple devices which is explicable given that these are the two most popular brand in the US. According to Maurice Klaehne, Research Analyst at Counterpoint:
In any case, OnePlus is pushing ahead with the 7 model already, registering not one or two, but three new model numbers for it. Those include a version that is eventually being called the 7 Pro with model number GM1915 and the alleged specs include 6.7" screen, Snapdragon 855, 8GB RAM, and a triple camera setup (48MP main +16MP wide +8MP telephoto).
The 5G-carrying version, however, maybe an offshoot of the 6T design with a rearranged camera rear, as you can see in the claimed case leak for it below. The new phones are likely not that far off, either, as in May it will be more than six months since the 6T was announced, and that's exactly when the OnePlus 7 is expected to be unveiled.
To recap, we may be looking at a 5G version and a base version of the current design, and a middle OnePlus 7 child, too. Where will the rumored pop-up camera go is anyone's guess at the moment. Oppo and Vivo which share the same production lines with OnePlus have most of their new phones featuring a mechanized camera of sorts, so OP 7 has every chance to score one, too.
Still, the moral of the story is that OnePlus won't be passed by the growing trend of releasing several models of one flagship line to fit every taste which defeats the brands reputation of focus a bit but we'll see how things pan out in the marketplace once those OnePlus 7 models get announced in a month or so.