Ego and/or a fear of failure; both of which drives people to their comfort zone. Hence the 'I'm a 3-4 coach so let me start by getting rid of all these good 4-3 players I inherited' mindset. The same mentality also drives the 'by the book' decision making (punting on 4th and inches at mid-field), which is most evident in baseball ('My 7th inning guy just retired the side on six pitches, but I'm going to pull him because he's not my 8th inning guy').
Yes it's maddening and yes it happens in all walks of sports and management. After we sold my last company to a big public company, they pushed out the founder/CEO and brought in their own CEO. Over the next year or so she kicked out 80% of the management team and brought in all her own people. None of them (including her) knew a thing about the business or the industry. 3 years and tens of millions in operating losses (that weren't there previously) , they sold the company off for tens of millions less than they paid for it. Oh well, another dead cap hit.