Jim, probably that the Toyota & Honda hybrids originated in Japan where diesel cars are not common, they then took off in the US (Prius) again not a big diesel market. Another factor is the added cost of a diesel version vs petrol and the reduced incremental improvement from the hybrid - more total cost and the ROI becomes less worthwhile.
I suspect that many hybrid buyers would be happy with the outright efficiency rather than counting fractions of cents cost. It will be interesting to see if the PSA or Volvo diesel hybrids do well.
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