[monopede]

That's very easy to explain. A low-pause GC requires a concurrent garbage collector. Writing a correct concurrent garbage collector is VERY tricky, so you usually start with something much simpler. Concurrent GCs typically also add overhead to the mutator (the user program), so it tends to be a trade-off of high throughput vs. short maximum pause times. As an example Java's G1 collector ("Garbage First") took a team of experts about 5 years to get correct.

OTOH, writing a standard (single-threaded) Cheney-style copying GC can be done in a few days. Actually, the more time consuming part for me was to get all the pointer information to the GC.