The Landsraad and the Spacing Guild wouldn't have stood for such direct intervention. The Empire is essentially feudal; direct imperial control of the Spice would have both destroyed CHOAM's profits (and hence the income of the noble houses) and deprived the Spacing Guild of the essential element of space travel. The reaction to overt movement against a noble house and towards dominating Spice production would have caused an immediate revolt of the Landstraad. Not to mention, if the Spacers knew large numbers of Sardaukar, why would they consent to transport them?

Better to control the spice through the Landstraad covertly.

The reason Maud'dib was able to effectively control the Spice and get the cooperation of the Spacing Guild was he already had an army there - he effectively took the spacers hostage before they knew what was happening, because he didn't need ships. Once he had control of the Spice the Spacing Guild was compelled because of their addiction to assist his troop movements.