I think this is a case where they feel they've developed the tech to an acceptable level where any more further investment would be a net negative. I know most people are ignorant on the topic and think NK is a joke with WW2 era tech but the facts say otherwise.
July 4th, 2017:
North Korea tested its first
intercontinental ballistic missile (ICBM) named
Hwasong-14 on July 4.
[45][46] It launched from the Panghyon Aircraft Factory 8 km southeast of
Panghyon Airport.
[47] It was aimed straight up at a lofted trajectory and reached more than 2,500 km into space.
[48] It landed 37 minutes later,
[49] more than 930 km from its launch site,
[50] into Japan's exclusive economic zone.
[51] Aiming long, the missile would have traveled 7,000–8,000 km or more, reaching Alaska, Hawaii, and maybe Seattle.[49][52][53][54][55] Its operational range would be farther, bringing a 500 kg payload to targets in most of the contiguous United States 9,700 km away.
November 28, 2017:
North Korea launched an ICBM from the vicinity of
Pyongsong at 1:30pm EST/3:00am Pyongyang time. The rocket traveled for 50 minutes and reached 2800 miles (4,500 km) in height, both of which were new milestones. The missile flew 600 miles (1,000 km) east into the Sea of Japan; unlike summer launches, the Japanese government did not issue cellphone alerts to warn its citizens. North Korea called it a
Hwasong-15 missile.
Its potential range appears to be more than 8,000 miles (13,000 km), able to reach Washington and the rest of the continental United States.[68][69] Much about the missile is unknown. The missile might have been fitted with a mock warhead to increase its range, in which case the maximum missile range while carrying a heavy warhead might be shorter than 13,000 km. Based on satellite imagery, some experts believe that North Korea may now be able to fuel missiles horizontally, shortening the delay between when a missile becomes visible to when it can be launched.
[68] The rocket is believed to have broken up on re-entry into the atmosphere.
https://en.wikipedia.org/wiki/List_of_North_Korean_missile_testshttps://en.wikipedia.org/wiki/List_of_North_Korean_missile_tests
That was their last test on November and by far the longest range they've ever demonstrated. a possible range of 7000-13,000 km. Enough to hit the entire continental US at the high range, and enough to hit Hawaii and parts of the US west coast at the low range. It looks to me like they're saying to the world "ok, now you know we can do this so don't even bother trying to invade and regime change here." Theres no need for further tests, they've already reached their milestones.