Nayeon does get a lot of promotion, but she is not the only one. She gets more than other members of Twice, but that's because she is the most popular member in SK amongst the GP.
Other members get solo projects, but it is true Nayeon gets more. It's obvious why though. JYP, the man, also uses her a lot to promote his own solo projects and work, so there's that too.
Her covering two Ariana songs is not really a big thing though. Jihyo also did a cover, and Dahyun did a piano cover as well as made a piano solo of Feel Special. So, I think there is opportunity there for other members.
In the end, I don't think it's really why is Nayeon the only one who gets solo projects? That's not the question. It's why don't other Twice members get more?
I would like to see other members get more opportunities, but I don't look at it as Nayeon somehow taking their opportunities. I'm fine with Nayeon having a lot of solo projects. Though of course I'd like to see other members be able to go into different fields as well.
I think the members who probably get the most solo promos are:
Nayeon
Dahyun
Chaeyoung