They are often GA aeroplanes ploughing into the ground with little available evidence.
And they are probably the ones that take the longest. In the days of CVRs, FDRs and QARs, it is pretty easy and quick to work out what happened with a big machine. If you don't have any of those, I imagine it could take longer to come up with a plausible theory of what happened. And of course speed of report is probably directly related to numbers of staff (that know what they are on about) available to do it.