Halleck had been suggesting that Buell send troops by water. Buell was not very cooperative. In August, Halleck dealt with another uncooperative general.
Halleck complains about Grant taking a long time, though, while Grant protests about the availability of transportation; were there ships available to take Buell? I ask this because it actually does matter - you can't move troops without transportation.
As of March 16, meanwhile, the orders to Buell were to move by land. I agree that earlier Halleck had been suggesting Buell move by water. Note that I say
orders and
suggesting because Halleck changes structure there.
But my point is that there's a lot of delay going on, both in messages (Halleck is regularly getting messages from both Buell and Grant days after they were sent, a thing which comes up again in August where Halleck is expecting replies to messages that his correspondents haven't even received yet) and in movement of troops; seventeen days after being ordered up the Tennessee, Grant reports that he hasn't yet finished disembarking. The cause of this is that there's not enough transports to move everyone in a single lift.
The same thing doesn't happen with Pope because Pope has enough transportation to move his entire fighting echelon, but he has to leave behind his sick and his transportation (wagons) to do it. It then takes about another week or so after Pope arrives before Halleck can say he has enough wagons to move his army.
Many of the American officers had experience in the Mexican war, so they weren't completely inexperienced.
But the scale is quite different. At that, look at the waterborne movement in Mexico - Scott sails in 1846 and doesn't land at Veracruz until March 1847, then of course it takes from March to September to advance to Mexcio City. Scott's entire force is about 20,000, which is within the scale that can operate by "living off the land".
Halleck, of course, is not Scott. He's a fortification designer and scholar; he hasn't handled any logistical or operational command before ending up as a senior general. Most of the operational and logistical experience in the Mexican-American War doesn't really go to the people who'd then be making those decisions in the ACW, though I suppose if you want to argue that Patterson showed great skill in those matters you might have a case...