It is more or less clear why a relationist would not believe in absolute motion (velocity) but why would a substantivalist believe in it?

Historically, various issues were intertwined by people like Leibniz (the paradigm relationist) and Newton (the paradigm substantivalist). For example, the question of whether space is a substance with independent existence is (at least apparently) a different question from whether there is absolute motion (velocity) or whether all motion (velocity) is merely relative. (One can see this when one thinks about Galilean space-time and notes that one could think of it in a substantivalist way even though it does not support the existence of absolute velocity.) Why is it natural to run these two questions together? Or, if it is not natural, what would make it natural? It is more or less clear why a relationist would not believe in absolute motion (velocity) but why would a substantivalist believe in it? Is there anything in the notion that space has properties independent of its material contents that suggests that motion will be absolute? Are there any other questions that get run together in the debate? And, what leads to those questions being run together?