I am currently adding the citations from the Royal Gazette to the district articles, mostly working from present to past. I have now covered all the district created since 1990, as well as all the upgrades of minor district to full districts from 1990. So I can now do a little statistics on these last 18 years. The most obvious fact is that the last time a bigger number districts were created was on July 1 1997, as well as the last upgrade was on October 1 1997. After that there was just one district (Wachirabarami in Phichit) in 1998 and one minor district (Wiang Kao in Khon Kaen) in 2006, and now the upgrade of all the minor districts in August 2007. 1990-1997 there were between 10 to 20 new (minor) districts each year, and a similar number of minor district upgrades. Altogether 202 districts were created in this timespan.
I can only speculate what is the reason for this hiatus, my guess is that it was actually a result of the Asian financial crisis which broke out in 1997, and the government tried to save expenses by avoiding to create new posts in newly created districts. In the 1920s and 1930s there was a more severe budget crisis, which led the government to abolish several of the monthon and provinces to cut expenses, as mentioned earlier.
Maybe a lot of new districts are in the pipeline now, catching up the 10 years of no new district. Maybe the already assigned geocodes I talked about earlier are the new districts to be created soon?