The third Friday of each month is an expiration for the monthly options on each stock. Stock with standardized options are in one of three "cycles" and have four open months at any give time. See http://www.investopedia.com/terms/o/optioncycle.asp
In addition some stocks have weekly options now. Those generally have less interest because they are necessarily short-term. Anything expiring on April 8 and 22 (Fridays this year but not third Fridays of the month) are weeklies.
The monthly options are open for longer periods of time so they attract more interest over the time that they are open. They also potentially attract a different type of investor due to their length of term, although, as it gets close to their expiration date they may start to behave more like weeklies.