And if we did find DNA (itself a monumental discovery), sequencing it would be the next most important bit of information we could want, since it might allow us to check if there is some relation to life on earth.
Is this a combo of a) the cost of the second question (i.e. the weight and size of sending the DNA sequencer) is fairly small, b) we're confident enough about question one that putting resources toward question two is worth the opportunity cost of asking a different question, or c) we have no other questions that we can reasonably ask for the same amount of resources?
Unless they find the place teeming with life, which nobody expects, it's more likely that anything they find hitched a ride from Earth instead of originated on the planet. So if they're not willing to rule that possibility out, they'll just end up causing decades of speculation and conspiracy theories.