This could be for several reasons.  If an event is “full” (the maximum number of people have already signed up), the RSVP buttons will be disabled when you visit the event page.

See also: What does it mean if the RSVPs are “Closed”?

Please note, required FTC Legal Disclaimer: HoustonPhotowalks uses affiliate marketing and advertising to generate revenue for our budget and the organizers who run the group. This money is used to pay fees, website hosting fees, classroom fees, teaching fees, donations to museums and venues, provide bottles of water when possible, compensate organizers for their time, etc. When you click links, the HoustonPhotowalks organization may receive a fee when you purchase a product or service.