whats the reason riot stopped "traditional" skins again?

They actually gave Gangplank a "traditional" skin, though they called it "captain", and it looks a little different.

because it detracted from the effort on updating all a champions other skins in a VGU. And would force them to dedicate extra resources to the champion and likely delay their update as they had to make essentially an entirely new skin.

This is the reason we didn't get a traditional Sion Skin, or a Traditional Poppy Skin.

None of the new updates really warrant traditional skins. Unlike Trundle when he straight up became an ice troll as opposed to a swamp troll.

They also said they felt like they could honor the older look of the champion in a better way.

Honestly, I think it would be cool if they continued to make traditional skins, but used the original models rather than coming out with completely new ones (i.e. using Gangplank's pre-rework model as a traditional skin), as a testament to the game's age and how far it's come.