The step up in size that Embraer took when they create the E170/175 and E190/195 aircraft was important for them and it proved to be a successful move. Both types did well and have achieved a solid market presence. With a new generation of technologies coming, Embraer decided to go for a significant upgrade to the type with new engines and other systems resulting in the E2 versions. In line with that, they decided to tweak the current design to create the E1 upgrades. This turned out to be a well-timed upgrade as it came at a time when a whole bunch of airlines were looking to up-gauge their regional feeder services. Embraer picked up a ton of orders.
The speed with which these jets have entered the US fleets is impressive. Both United and American signed new deals for service with these jets and now you can regularly see their E175s feeding in to large airports. Alaska has gone a similar way (using Skywest much as United has) and their fleet of E175s is starting to grow. The E2 has now had its first flight in the larger E190 form but the 175 will follow in a few years. The E170 has been dropped from the line at this point. I imagine we will see even more of these jets as they will dominate this seating range which Bombardier seems to have ceded as they focus on larger jets.