Beyond the immediate causes as described in Wikipedia:
- Early on, it was decided not to build a test stand for the first stage, as this would take too long (I've also seen a claim that a test stand would tie up too much of the USSR's concrete production, but haven't been to able to find a source for this). Instead, they planned to do a series of launches with dummy payloads. 14 tests were planned, and it was expected that a lot of these would lead to launch failures/explosions.
- The death of Korolev slowed down the program, and the new director wasn't a fan of the N-1.
- There were quality control issues (one of the failures was caused by welding slag left behind in a propellant tank).
- The engines were designed for single use, to the point where they couldn't be test-fired prior to installation in the stage. They 'solved' this by building engines in batches of 6, testing 3 of them and if they passed, installing the remaining 3. A follow-on design that fixed this was scheduled for the 5th launch, IIRC.
L3 - Russian manned lunar expedition. Development begun in 1964. All hardware was test flown, but program cancelled in 1974 due to repeated failures of the project's N1 launch vehicle. Status: Cancelled 1974. Gross mass: 95,000 kg (209,000 lb).