The change of base rule says
[tex]\log_b(x) = \frac{\log(x)}{\log(b)}[/tex]
If b = 1, then we will have
[tex]\log_b(x) = \frac{\log(x)}{\log(b)}[/tex]
[tex]\log_1(x) = \frac{\log(x)}{\log(1)} = \frac{\log(x)}{0}[/tex]
which is NOT possible. We cannot divide by zero. So this is why b = 1 is NOT allowed.