Let s start with what ascension is. It is a process of raising your vibration to embody higher-energetic states of being. That which you refer to as dimensions are frequency ranges. These ways of being are determined by the frequency range.
Different frequencies require and create different conduct and different realities.
Most adults on the planet at this time were likely born into a physical body set with a vibrational range of 3d reality. Most likely you did not think about it much until you became older. Then the way of being is deeply ingrained, making it more difficult to change. Back to the dimensions. As energetic beings in One unified field, the more people who raise their vibrations to the frequencies ranges of 4d, 5d and so on, the easier it becomes for everyone else.