You say: "A well-known and specific example is that Leibniz is less well regarded than Newton for his calculus".
I think this is just incorrect. Leibniz version of calculus lead to an explosive development
of calculus in continental Europe. Think of l'Hopitale, Bernoulli's, Euler and many others.
While calculus in England experienced stagnation in the period after Newton.
Exactly for the reason that English mathematicians ignored Leibniz's version.
Name me ONE English mathematician of 18 century in the class of Bernoullis, Euler, Clairaut, d'Alembert etc. Only in 19-s century English mathematicians overcome Newton's dominance and accepted Leibniz notation. Which we are all using now.
EDIT. Of course I agree that Newton was one of the greatest mathematicians of all times.
But nevertheless he certainly had some negative influence on development of mathematics, mainly in England, but not only in England. On my opinion, this happened for three reasons:
One is that he was so much superior to everyone else. Not surprisingly his cult was formed. No one could contradict him. (Think of the history of Optics, where his wrong opinion was dominating for almost 100 years).
His domineering personality. He suppressed and ruined great people around him (Flamsteed, Hooke and others). He staged this shameful "priority dispute with Leibniz".
He did everything to put Leibniz down, to destroy his reputation (like he did to Hook). And he partially succeeded.
And this discussion is another evidence of this.
Finally he was reluctant to publish his results, while Leibniz did really a lot to promote calculus. He created a journal. He had students, etc.
In his life time Newton was surrounded by really great people. And look how it changed after his death. Read the history of the Royal society in Wikipedia, to begin with.
EDIT. Newton wrote once: "If I saw farther than others, this is because I stood on the shoulders of giants".
Recently I read a paraphrase of this: "If I did not see as far as some others, this is because giants stood on my shoulders" :-) I think Flamsteed or Hooke would be justified if they said this. They were the giants on whose shoulders Newton stood. And what did he do to them?