The reason ?
Because ;
1/ Zenith created more than 500 calibers since the birth of Zenith. yes, more than five hundred. here you see a lot more info, not all of them but a lot.
http://zenhitsite.monsite-orange.fr/page7/index.html
2/ Brands like Parmigiani, Rolex, Panerai, Ebel, and several others used Zenith movements
3/ People only think of the El Primero and maybe the Elite but know that the El Primero has dozens of different calibers. I think it's a shame Zenith calls all of these EL Primero but out have El Primero's with flyback, splitseconds , date, calendar ( annual and perpetual), 2 or 3 subdials, big date, tourbillon, repeater, ... Most other brands would give every single movement another name. So tell me, does Zenith have 1 or 15 movements today ?
4/ Look at their history, the Calibre 135 is still today a VERY sought after movement / watch. Just lik the Longines 13 ZN of CH30. Or what about the legendary 5011K which was the most accurate movemetn EVER tested back in 1967 (in Neuchatel) , today it might still be the most accurate. FYI, you can find an extremely limited watch with this important movement , the 250 LE Montre D'aéronef type 20
5/ like mentionned before, they make all the complications, like split second, perpetual, tourbillon, repeater and their level of finishing is the same as JLC, so tell me why I coulld not compare these 2 ( I have both)
And there are many other reasons. The only thing where Zenith did not succeed untill now is good marketing. They have everything they need, inhouse legendary calibers & a great history but somehowe brands like iWC got more attention. And Mr Nataf wasn"t their best move too.
Like said before, since 2009 Mr Dufour ( CEO) is doing a great job. Give him some time and people will respect Zenith more and more.