Isnt the 737-600 the closest thing boeing has to an RJ?
No... the 717-200 is a true "RJ." The defenition of RJ
has been blurred in the last few years, it use to mean a 50-100 seat aircraft that would fly less than 1000 nm. The 717 is of this vouge, it doesn't have long-range capability but it's designed for short, rapid cycles with little down-time. The E170-E190 are more for long-ranged point-to-point flying than they are for true regional flights.
Airlines are trying to replace some mainline aircraft with less expensive "RJs," which is why the E190 is attractice, it has the range to fly many sectors a 737/A320 would fly. The 717 is still an excellent regional aircraft, but is either too large or too heavy to fit the newer niche created by the E170/E190