The main reasons behind this discrepancy are production goals and concerns. The Galaxy Class was designed for long term, deep space exploration, but most of TNG sees the ship involved in "local" dealings of a diplomatic nature. This allowed the writers to world build more than sticking to the original premise, which would have seen the Enterprise-D keep going to "see what's out there."
I personally think the focus on diplomacy/politics/etc. made the show far less interesting as it evolved. I wanted strange new worlds and new life and civilizations, not revisiting previously known civilizations or species that were just new to us and not the era. I enjoy many of these episodes, especially the Klingon stories and just about every time the Romulans show up, but I was always aware that the Enterprise-D was made for more.
I personally think the focus on diplomacy/politics/etc. made the show far less interesting as it evolved. I wanted strange new worlds and new life and civilizations, not revisiting previously known civilizations or species that were just new to us and not the era. I enjoy many of these episodes, especially the Klingon stories and just about every time the Romulans show up, but I was always aware that the Enterprise-D was made for more.