I am basing this on the fact that moving a large army in a spring offensive required one critical item, Good Roads. The launching of the Union Army Spring offensive on April 2nd and the quick pursuit of Lee's retreating army meant that the roads were in good shape and dry. In the accounts of Lee's Retreat from Richmond I do not remember many references to muddy roads being a problem. This would have meant a good period (maybe a week or so) of warm, dry, sunny weather before the Union army would have committed to an offensive which could not have been followed up by the large amount of Cavalry they had on hand. That Cavalrys swift movement was the key to capturing Lee's Army.