1) too many people don't understand why they probably should use snapshot isolation (or stricter) and don't understand what guarantees they don't get when read committed is used
2) it's not the default, defaults matter, a lot
3) it makes transactions spurious fallible when the db can't make sure that committing two parallel write transactions won't brake the consistency guarantees, a lot of frameworks don't have the right tools to handle this, people don't expect it, it can make in the transaction interleaved interactions with other systems harder, etc.
(as a side not I assumed you meant REPEATABLE READ when you said snapshot isolation, as it's the least strict isolation level which uses snapshot isolation)