In reading posts from people whose kids have skipped grades, a common thread is that they develop ADD or problems with executive functioning, etc. Obviously, many kids who skip do very well and it's a great option.
However, there are probably also quite a few kids who emotionally are not able to handle the skip. They may handle it as you skip them into second grade but then when they are young 7th graders (etc.), they can't handle the huge homework load, the need to take notes quickly, the importance of filling out boring worksheets correctly.
I've asked this before- I wonder how many grade skippers end up in the top 10% of their high school class by GPA? (Not to say that GPA is the final metric to measure someone but I would guess that some grade skippers lose out ultimately by skipping).