There are two answers to the question:

The first one is:  Yamaha had a Supermotard back in 1988 already. Yes, the TDR 250 was one of the first machines to combine street performance with offroad characteristics.

The second one is:  Supermotard has always been a kind of 'niche' market in Europe. It did not become a major category like 'Adventure Touring' that was initiated by Yamaha's TENERE in the middle of the 80's. As a result, there are not many 'Supermotard' machines of the production line on sale.

Usually, enduro bikes are used as a basis and then modified with cast wheels, wider tyres and bigger brakes. For real supermotard racing, the WR426 hard-enduro offers a good base with it's powerful, compact and lightweight engine. For street riding, customers sometimes modify their XT600 or TT600's in the same way --- with very nice bikes as a result!

Time travel to the eighties
Let's have a time-travel back to the middle of the 80's. When XT 600 Tenere was boosted by the success of Paris-Dakar Rallye, the Product Planners had another concept idea in their minds: to combine the suspension comfort, the agile handling, the high position and the controllability of an enduro with the performance and the stability of a street bike. The target was a motorcycle which would be very enjoyable on small tiny roads and easy to control even on extreme cornering angles. So in other words, a fun-bike for narrow pass-roads or for changing surface conditions, where conventional supersport machines require hard work from the rider!

As early as 1985: the first prototype
Yamaha Motor Europe made up a prototype machine with a 900cc bore up version of the XS 650 twin engine. This air-cooled classic twin was mounted in an XT 600 Tenere framework with a special front fork and rear suspension plus wide street tyres. Yamaha testriders together with the product planners tested the machine in real conditions in the Alps and were all of the same opinion: a great bike to enjoy a sporty ride on mountain roads. The enthusiasm however was cooled down after market research. The focus of most sports riders was on the upcoming supersport road machines, like Yamaha FZR 1000 or the earlier Suzuki GSX-R 750.

Engineers see benefits in Supermotard
But a bunch of planners and engineers in Japan still believed in the benefits of combining road and enduro virtues. In particular for the tiny small roads in Japan, this concept was supposed to attract sporty riders. The TDR 250 was the answer. This unique bike combined the high performance of Yamaha TZR 250 supersport 2 stroke twin with the agility of an offroad-derived tubular steel chassis. Shorter wheel travel and the massive disc brake from TZR 250 made the TDR very competitive on small roads. With special semi-offroad tyres, the bike was also capable to handle light unpaved roads. It had it's debut at the Tokyo Motor Show in 1987. The press text mentioned: "Combining nimble handling with instant acceleration on paved roads, this lightweight ripper will write a new chapter in motorcycle history". The bike sold well in Japan and in Europe. 50 percent of the TDR's went to France, where the Supermotard trend was the strongest. With the decline of 2-strokes, TDR 250 reached the end of it's life-cycle a few years later.

The TT600 4-stroke single was later considered by the product planners to be a good base for a revival of Supermotard. Again some prototypes saw the light of day, but the situation had not changed a lot: real Supermotard, (or 'Superbikers' as they are called too), were still a niche market. The extreme variants like KTM Duke or Husqvarna NOX attract a special group of riders, but could not appeal to a wider audience like the more versatile concepts such as BMW F 650, Suzuki Freewind or Aprilia Pegaso for example. These bikes are more touring and practical oriented rather than sports oriented.

Sven Ermstrang, Product Planning Manager at Yamaha Motor Europe: "To be frank, in our hearts we favour a more 'emotional' approach for future single cylinder machines rather than a 'logical' approach. We are still considering the bandwidth from a conventional enduro like TT or XT 600, to more supermotard or street oriented concepts. Still we believe that the strong engine character of a single cylinder has a special attraction, a special value and we would like to define in which type of machine the customer would like to see that."