you get stories like this that go on about how incredibly sensitive DNA testing is, but then when something happens to you the local plod claim they can't prove anything. Given how inept this pair seems to be it seems unlikely that they were dressed head to toe in protective gear to avoid leaving dna behind.
BBC News - The strange case of the 'time travel' murder
No idea if this is some "economic" decision to not spend the money getting the tests done etc. or what. Personally would expect that the costs for not aggressively following these things up will end up larger as they are likely to continue the "habit"