Yes, I get the impression that with the advent of iDeCo there's been an increased interest in mutual funds in general and a lot more competition. Several of the other funds also decreased their management costs significantly.
The eMaxis slim series is basically mufg's reaction to this, their maxis series index funds were just not competitive in price so they pushed out a new series.
Other than the cost I'm not sure what is different, but I suspect they somewhat limit their tracking(so it won't be perfect) to reduce trading costs and keep the price down. It's worth noting 1550 is also an mufg product and it's part of that older series(Maxis)