Market for vegan burgers with better health effect than non-vegan burgers is vegans+health concerned non-vegans which is much larger than vegans alone.
Market for non-vegan burgers is always non-vegans. Being more healthy can affect the standing of particular non-vegan burger among other non-vegan burgers, but is unlikely to attract many vegans (which is an order of magnitude smaller market anyway and couldn't have high impact on the larger non-vegan market).
It's a false equivalence. Burgers are not healthy - and very, very few people are looking for 'healthy burgers'. People are looking for 'tasty burgers', which is why so much effort is put into replicating the flavour and texture of meat.
(†) I believe if they were health conscious, they would it less meat, but would not be vegan.