Fertilization

This may be a dumb question, but why can't you get pregnant once you have ovulated?  I am getting ready to do my first round of IUI and am instructed to come in the day after I get a positive OPK, per my doctor  because I will ovulate within the next 24 to 36 hours.  Does the egg become impermeable if it is released and there is not a sperm there to fertilize it as soon as it is released?