Because he commit suicide. He didn't just die, his death was tragic and a shock to the studio. After the Show ended they had continued on making cd dramas for the content however upon his death they decided they did not wish to continue that arc any further. And rehashing hikaru is ok if it is using the same VA's lines and dialogue, however they have created no new dialogue for him. It is all just rehash. In cases where the actor dies naturally the japanese are ok with replacing him, it is just when it is in bad taste to them culturally that they will not. If you study the japan culture you will understand why they choose not to go and replace his VA.
And as VF-15 so nicely showed you in his picture the meltrandi ships look alot like the sdf-1's design. also their ships open their main cannons horizontally just like the sdf-1. The zentradi ships open their main cannons vertically. Watch the movie and you will see if you watch it closely. In closing, to repeat, all arcs after DYRL use the DYRL universe, not the universe fro SDF Macross.