"The first Surface Pro was a sizable 2.0 lb monster tablet which packed Ultrabook class components into a chassis that was over 0.5-inches thick. It was powerful, but heavy and the 16:9 form factor was not ideal for a tablet."
I think the extra size and weight is totally understandable given that it's a full-blown PC, but Microsoft's marketing message is that this is something that will replace both your laptop and your tablet with no compromises. They've used that phrase a lot (no compromises), but to a heavy tablet user the extra size and weight in tablet mode would've seemed like a huge compromise. 1/4 lb isn't much of anything in absolute terms, but in relative terms for that product category the hyperbole is probably warranted.
A laptop's weight is only relevant when you're transporting it, otherwise it sits on some solid surface and how much it weighs is fairly inconsequential.
A tablet's weight is felt in most use cases during regular use. A lot of people hold tablets in their hands while they're using them rather than set them on some solid surface with a stand - this makes them a fair bit more weight-sensitive than laptops.
Same reason why the 16:9 form factor is fair criticism - it makes the tablet really awkwardly balanced when used in portrait mode, something that isn't an issue with laptops which can only be used in one orientation.