The US military command divided up South Vietnam into four zones numbered I to IV, with I in the north bordering North Vietnam and IV in the south (essentially the Mekong delta). I'll dig up my source material when I'm not on my phone, but from memory I can tell you something like 50% of all US fatalities were in the 5 provinces that made up I Corps, and half of those casualties (25% of total) were in the two northern most provinces, which include such infamous locales as Khe Sanh, Hue, Con Thien, and the A Shau Valley where Hamburger Hill was located. This zone was dominated by the presence of the Marine Corps which deployed it's troops there almost exclusively - which is reflected in the Corps' roughly 25% total casualty rate over the duration of the conflict.

As to why North Vietnam focused on that region, one simple explanation is geographic proximity. They had to ship supplies down through Laos via the Ho Chi Minh trail, which was bombarded frequently by allied aircraft and MACV SOG Recon teams. This made moving troops and supplies simpler from a logistics standpoint simply to have them arrive right at the fight and not be worn down by months of travel to the southern front near Saigon. I don't have the numbers offhand, but I've read that the ratio of PAVN (aka NVA) regulars relative to local Vietcong guerrillas was significantly higher in the north. The better training and equipment of the enemy troops up north would also reflect on US casualties, by the way. The other reason to focus on northern South Vietnam was the presence of the city of Hue, the former imperial capital from when Vietnam was unified. This was a significant city to take for both practical and symbolic reasons, and was arguably the main effort of the PAVN during the Term Offensive. It was also briefly captured and besieged during the Easter Offensive in 1972, and it's capture was actually the long term objective of the PAVN units that were destroyed at Hamburger Hill.

Now, to counter all of this enemy activity, the US deployed far more troops to I Corps - both the 1st and 3rd Marine divisions, and at times, often simultaneously, the 1st Cavalry division, 5th Infantry division, 101st Airborne division, part of the 82nd division, and numerous Special Operations units. In other words - it was a target rich environment for both sides. Additionally, despite their supporting fires paling in comparison to the US capabilities, the North could often hit US troops or bases with artillery from Laos or possibly even over the DMZ - an major advantage that was lacking or absent farther south.

Wrapping up until I can get home and find my exact sources, I'd like to mention that of the the 3 zones, both II and III saw extremely heavy fighting at times - the Dak To hill fights of 1967 (II Corps) were there, and were arguably the bloodiest campaigns the US military engaged in during the war. III Corps, conversely, was located around Saigon which was naturally a major PAVN/VC target and by extension an important location for the ARVN, US and other nations to defend. Eventually some of the bloodiest battles of the war would take place there, including the siege of An Loc in 1972, after most US ground troops had withdrawn. The only region which didn't see particularly heavy US casualties was IV Corps in the Mekong delta.

The American death toll was biased to deaths in ones and twos in small unit engagements and encounters with booby traps and land mines -- but added up with dozens or scores of those engagements and encounters around the country every day. Large battles were (as a rule) a bad idea for the Viet Cong and NVA ... because the American military was very good at large battles and fairly bad at counter-insurgency. The Tet Offensive had its historical impact not because it was an NVA or Viet Cong victory (it was a huge defeat in tactical terms), but because it gave lie to the story the two years of high-tempo, high-overall-casualty counter-insurgency had significantly degraded enemy capabilities.