I'm assuming you're speaking of the H-1B visa, based on the April-only points. The application period begins in April and there is a limit on the number of these visas that may be issued in a year.
While technically these visas can be applied for throughout the year, there is a rule in place that stops applications from being accepted once the limit has been passed (or some buffer above the limit, maybe). After the first week in April, if this threshold has been passed, applications will not be accepted until the following year.
This visa has hit this limit quickly every year for many years now. In part it is due to popularity of the visa. It is also due to those filing knowing they must do so in the first week of April in order to not be cut off.