The NSTB are not "drip-feeding" - they are reporting facts as they gather them in accordance with their procedures. Each accident will be different and require different actions. This accident is unusual in that there is so much recording equipment on board, thus hard evidence will come quicker and faster. There will still need to be a full analysis which will take time and the NSTB have clearly pointed this out.
There is an excellent article in the Financial Times regarding the commercial, marketing and
PR motivations and issues - I won't post the link because it won't work if you don't have an FT account, but it sums up the situation very well indeed.