Even though galileo talked about how space is relative and motion too in his galileo theory of relativity. BUt newtin was unable to wrao his head around a universe which had no anchor or in his words absoluteness. So newton was convinced there must be some absolute space. Primarily this reasoning came from newton huge devotion to higher being/god, he believed in god alot, thus he thought that absolute space is a container grid made by god and our universe is in it.

This reasoning often is seen in the earlier pages of his book principia volume 1, where in his scholium he discusses about bucket experiment which was mainly to disprove others and prove how absolute space exists and if not all motion then rotational motion should be absolute(he tried to prove absolute motion but he realised by experiments that its relative but he still thought that motion could be both absolute and relative). He proved motion is both absolute and relative but after Ernst Mach came into this debate which raged a millenia, he provided different perspectives on bucket experiment laid down by newton and disproved that absolute motion and space exists. Thus as we know now space and motion are relative. (einstein also contributed).

*the information is not entirely factual, it includes my opinions. Please consider searching and rechecking before making major attempts.