Might I ask how you know this? Or are you just surmising based on the qualities that you are able to identify as a viewer? How do you know that the presenter does not possess qualities which are not apparent on the screen? Might they be reliable, punctual, tolerant of long working hours, organised, co-operative with other employees? Might they be part of the production or research team?
What is the alternative? Experts? How, for example, would you find an expert to present the One show, a programme which is, after all, about absolutely nothing?
I don't know the answer to the question I have asked you, beyond having a close relative who held a relatively senior position at the BBC up until about 15 years ago. The only insight I have is that things are sometimes far more considered than you might expect, and politics play a large part (see J.Savile Esq)
You are right that Wallace was given the job. Lots of jobs in fact over almost 25 years. It seems unlikely that his overbearing boorish behaviour was limited to Masterchef. Perhaps the question is not 'how did he get the job', but 'how did he keep it'.
As an aside, here is an excerpt from Wallace's Wiki page on his marital history:
"Wallace married his first wife, Christine, in 1991 but their marriage only lasted six weeks. He has two children, Libby and Tom, with his second wife, former pastry chef Denise, whom he was married to between 1999 and 2004. Wallace then married biology teacher Heidi Brown, in 2010, his third wife who was 17 years his junior, after meeting her on
Twitter, in 2010. But the two went their separate ways after 15 months."
Read into this what you will.