You use 'most' when you are talking about some large part of an even larger group.
In sentence 1, we cannot say 'most reasons', because there is not a large group of reasons we can measure against. This would be correct.
Of all the reasons given for speeding, most of them involve lack of attention.
In this case we have the larger group of all reasons and the group included in that main on of reasons involving lack of attention. This was not true in sentence 1.
In sentence 2, the total group is the group of all children who watch 6 hours or more of TV. The smaller group is the number of children who lack self esteem - a smaller group than the total, but most of it still.