While we are all film buffs here, and we are all open to watching movies from all decades, when we talk about the top 100, we are doing a combination of favorites lists.
Part of what is a favorite is what communicates to us, personally. Potentially, any decade's movies could communicate to us, strike us, but the likelihood is higher that a movie that is closer to our cultural heart will do so. In other words, most of us are people of this era. The cultural assumptions and communications of previous eras may strike us, but more movies of this era should communicate to us.
The other fact is that there are simply more movies available from more recent decades than earlier.
Frankly, the FS100 covers quite a broad spectrum. Broader than any modern list should be without some leaning the other way.