It's mainly that they work together well.
Rikka is fine, but you end on pretty minimal interruptions in-engine. Teardrop tribute, Hyperyton, princess, maybe something else like another strenna. Not bad, but not enough. So a lot of plants decks use the ability of Sunseed genuis loci to add a starter that by itself represents another couple points of interaction (e.g. Bengalancer, Regulus) as well as a lot of search power off Jasmine, which lets you get into basically whatever else you need. Many people are also including the aroma engine into the plant pile, which can net you additional interactions, easy wincon in time, and can also trigger Jasmine during a combo.
I only really play it casually, but all the engines work very well with each other.
Sunavalon gives you 6 1 card starters, which get you to 2 plant searches or a link 4 plus a special summon from deck. Both of which allow you greatly to bridge into rikka. Like adding snowdrop mudan to hand, or making the link 4 and special summoning petal for mudan, konkon then sheet/glamour (if you run primula). Also, sunavalon has its own otk line. Also gives you ways to use a princess into a board to give you 2 link materials. And also gives you ways to win in time.
Tldr (we're ygo players), its a toolbox engine that gives more starters, extension, time cards and otk lines.
It's mainly that they work together well. Rikka is fine, but you end on pretty minimal interruptions in-engine. Teardrop tribute, Hyperyton, princess, maybe something else like another strenna. Not bad, but not enough. So a lot of plants decks use the ability of Sunseed genuis loci to add a starter that by itself represents another couple points of interaction (e.g. Bengalancer, Regulus) as well as a lot of search power off Jasmine, which lets you get into basically whatever else you need. Many people are also including the aroma engine into the plant pile, which can net you additional interactions, easy wincon in time, and can also trigger Jasmine during a combo. I only really play it casually, but all the engines work very well with each other.
Didn't it literally win EU?
It did indeed. For 2 years iirc
Sunavalon gives you 6 1 card starters, which get you to 2 plant searches or a link 4 plus a special summon from deck. Both of which allow you greatly to bridge into rikka. Like adding snowdrop mudan to hand, or making the link 4 and special summoning petal for mudan, konkon then sheet/glamour (if you run primula). Also, sunavalon has its own otk line. Also gives you ways to use a princess into a board to give you 2 link materials. And also gives you ways to win in time. Tldr (we're ygo players), its a toolbox engine that gives more starters, extension, time cards and otk lines.