^^^
There are a lot of Japanese fans who ship them. It's because they have good chemistry and they are really close.
Here some reasons
They chat everyday on line.
For some reasons from their stories and from others hologirls they seems to be always (almost) together(?)
Miko seem to go often eat at Sui's house.
They often mention each others name in their streams.
When chat said Sui hate pink, Sui said she doesn't and then she mention Miko. I mean she could said Aqua, her sister or her favorite plushie but nope, the first person she thought was Miko.
I wasn't watching hololive but I heard from their fans that Miko had a lot of pressure during the sport festival but the only person she ended up to open up was with Suisei. (Miko used to be the type to hold keep her problem to herself)
They have duo mascot. Takoyaki and Maguchi.
Miko promote Sui songs a lot and often sing them.
During rust Sui learned how to play Miko song using an instrumental game.
A few years ago Marine asked a few hologirls to write a profile and Sui wrote that she want to get close to Miko which later Miko said on stream she was very happy.
It's very easy why people ship them especially if you understand Japanese or follow the micomet community.