Because -XX:-DisableExplictGC enables Systsem.gc() to work. You need to use -XX:+DisableExplicitGC to turn off calls to System.gc().
If you read the column heading for the options table on the JVM option web page at http://www.oracle.com/technetwork/java/javase/tech/vmoptions-jsp-140102.html, you will see that is says "Option and Default Value". Thus by default DisaableExplicitGC is off, you need to use the plus sign to turn it on.