The main problem is that he was simply not up to the job.
I am an avid F1 Follower and I lost count of the number of times, during Legard’s tenure, when I noticed things happening in the race which could have done with a bit of explanation. Instead we’d hear him prattling on about how the Sauber team’s light-bulb changer’s partner gave birth to a baby boy the previous Thursday. It was left to Martin Brundle to do the proper business for viewers.
It is not too early to judge Martin’s commentary. He has been regularly commentating since 1997 when he started alongside Murray Walker. Until this year he had been the number two commentator but has now rightly (and belatedly) stepped up to pole position. He is without doubt currently the most knowledgeable and competent commentator in any sport. Tellingly he survived the sport’s move from ITV to BBC (as did Murray Walker in reverse) and he and his new “boy apprentice” David Coulthard (whom he also manages) make a superb race commentary team.
I believe Legard transferred from his role as F1 commentator on Radio 5 Live when BBC gained the TV rights a couple of years ago and now fronts something called “The Football League Show”. F1 is a highly technical sport and viewers need something more than a journeyman journalist to help them follow races. Jonathan Legard (and to a lesser degree his predecessor James Allen) were simply not up to that task.