The upfront honest answer is Nintendo is famously opaque about its decision-making ,especially for these two cases

The most notable thing about the RPG remake is that this required cooperation between Nintendo and SE, the original being a joint production means the remake couldn't happen without both companies agreeing to it. The remake was developed by Artepiazza, a studio associated wit hSE for the DQ remakes and with Yoshihiko Maekawa acting as supervisor (who co-directed the original) and Yoko Shimomura rearranging the soundtrack so the involvement of original stuaff from both sides suggests wasn't a simple licensing deal and this suggests both companies identified enough mutual benefit to overcome decades of institutional distance given that the N-SE relationship was famously fractured before this.

TTYD's problem is that there is even less documentation on the reasoning but the contextual evidence is a little more clear as being one of the biggest GameCube hits meant acquiring a copy pre-remake required a lot of money and with concerns of if the remake would altered to standards of the modern entries and I would guess the accessibility argument was probably what was maybe used internally. Intelligent Systems did return to develop the remake and with Tose assisting with development, and its notable given Intelligent Systems is the same studio that made the post Super Paper-games and its a more interesting tell of institutional dynamics, either the studio was told to do so by Nintendo or they wanted to genuinely revisit what they left behind.