Because one of the biggest point of Nates work in 2008 and 2012 was about how in the American electoral system, state polls were far more important than national polls. Obamas campaign literally ignored the national polls because they were going in so hard on battlegrounds - heck, remember how badly Gallup missed the mark? They had Romney winning. But people who followed states polls knew this wasn't happening.
Nates model this time round has been tuned to be too sensitive to both national polls and 'trend setting wave polls' in other states. It's constantly looking for a shift in 'momentum' for a candidate, so if it sees an upward gain for Trump in Utah, it thinks there's a national tide and so his chances go up in California (for example). It's actually doing the *opposite* of what it should be doing - it's treating state polls in locked states as mini-national polls and changing everything everywhere based on them.
In 2008 Nate had a very good point, in that states aren't independent of each other, and that (for example) you can't win North Carolina as a democrat without already having won Virginia. So if the polls are showing you winning North Carolina but not Virginia, be suspicious of the polls. But now he's taken that too far, and everything is too interlinked and too sensitive so that slight changes on the other side of the country shift things too much.