Earlier quoted context omitted.
Probably because while he may have invented Javascript, many people have contributed to it, so that's not really the same issue. We should assume there might be racists/biggots/etc working at Google on Chrome, at Apple on Safari and at Microsoft on Internet Explorer, too. That's not the point. The issue is with Mozilla placing him as their executive leader; the endorsement that makes and the message that sends to the…
You can argue the exact same thing for mozilla and firefox: Brendan did not create firefox/mozilla, he's one of thousands who contributed. People can have their own personal beliefs and as long as that doesn't effect the organization's open and inclusive policies.
Brendan was an employee of Mozilla when he made his Prop 8 contribution - that's fine, that's his right and as you point out people are entitled to their own personal beliefs.
The issue is who a company/foundation/organization decides to be it's leader and the values that leader will exert upon the company/foundation/organization and the products it produces.
To be clear, my beef isn't with Brendan Eich (although I'd disagree with him if I met him), it's with Mozilla for selecting him to become CEO.