Why is fixed better on wet?

More control over the wheels. No free wheeling and so you tend to slow in anticipation rather than suddenly. Where it doesn't work as well is cornering over slippery surfaces such as wet metal manhole covers, when the rear wheel seeks purchaes which it would not need to if you were free wheeling.

This last bit is true to a degree...if you had your rear brake applied it owuld most likely be worse than cornering on fixed.