Frederick William IV famously refused the Frankfurt Parliament’s offer to reign as German emperor, despite the fact that he was pro-unification. Wilhelm I, the man who would later become the first German emperor, likewise opposed Bismarck’s plans to orchestrate wars with Austria and France to achieve unification—for example, he acceded to French demands and withdrew a Hohenzollern prince from the Spanish throne.
Why were the Prussian monarchs against a German pan-nationalist movement that would bring more land and glory to Prussia?
Frederick William IV and William I had a conception of power, and a conception of Prussia, that was fundamentally conservative and rooted in the belief that political legitimacy is derived from the historical nobility, not from the popular will. They both wanted Prussia to lead a united Germany, but saw that unification occurring through the assent of the various German kingdoms, not through liberal, democratic means.
This explains Frederick William IV's "crown from the gutter" remark. The Frankfurt Parliament was a liberal institution; seen as an instrument of popular sovereignty, a product of the revolution of 1848. For a conservative monarchist like Frederick William, it was deplorable, and he would never accept a crown with legitimacy from such a base source. "The gutter" was how he viewed the Frankfurt Parliament and everything it represented.
William I's reasons are similar. u/kieslowskifan summed up his reasons very concisely in an answer from a few years ago that didn't get much attention, so I'll quote it here:
While Wilhelm I was not reluctant to be at the head of a Prussian-led unification drive, he was reluctant to assume the mantle of Kaiser in 1870/71. There were several reasons for this. For one thing, as a staunch old-guard Hohenzollern, he associated the Kaiser title with the old HRE and the "repression" of Prussia in the eighteenth century. Wilhelm I also saw these Kaiser titles as schemes of his heir who had liberal tendencies. This spoke to the larger antipathy Wilhelm I held towards modernity and its politics. The Prussian King could understand Bismarck's "Iron and Blood" methods, but he never really grasped the importance of parliaments and constitutions. The new imperial title was not one that came out of a distant past, but one created and sanctified by newer institutions. Wilhelm I was a man who did not take such entities seriously, so he was in no rush to assume a new title that would supersede his older, more pedigreed, one. Bismarck maneuvered Wilhelm I to accept the Kaiser title by having it be a gift from his fellow German monarchs.
I highly recommend the book Iron Kingdom by Christopher Clark if you want to get into the weeds on Prussia at any period, including German unification. It's fairly long and dense for a "popular history" book but very accessible to anyone with some historical literacy.