Because with alts we know what they are doing and planning. Bitcoin cant come to a decision.
Because bitcoin is so big truly decentralized, or maybe correct word would be: fractured, to the point that effectively managing its upgrade became pain in the ass.
On the other hand most altcoins are small project where developers can enforce their vision and shape the coin as they please.
Because no one care to the point to oppose developers and fractions supporting different solution within the community are not existent.