The answer is simple, the Nike Blazer is a narrow shoe and is pretty well-known for it, the mouth of the shoe where you insert your foot isn’t very wide.
This also means you more often than not are going to have to really loosen the shoe to get your foot inside. The high or mid-top version of the Nike Blazer is not a shoe you simply slip into.
However, to address this problem of the Nike Blazers being so hard to put on, a newer version of the Nike Blazer with a stretchy heel was released which is in fact a lot easier to put on.