I'm guessing Nikon was selling mostly to mariners and the 7x50 is more in-demand vs 10x50 for them. And they make 10x70 and 18x70 for the astronomers. Fujinon has been making the big heavy 10x50 marine porro.It's odd isn't it? 10x50 was a pretty popular format (at least in Europe) with lots being made by Zeiss (both), Soviet manufacturers and IIRC some other Japanese manufacturers as well. Yet Nikon never attempted eg. an E series 10x50. I wonder why.
And Zeiss....no more porros from Zeiss...no more 10x50 from Zeiss....no more 7x from Zeiss......too bad! Although I'm sure they would say the 10x54 HT is their version of 10x50....they do weigh about the same. Considering how long Zeiss made porros though, I'm sorry they don't offer one or two for tradition and pride in the design