The newer Minecraft versions bring with them a lot of new content which allows for CubeCraft to differentiate from servers that are still stuck in the past - cough cough Hypixel. Block palettes are that much better and more vibrant for building, new items and weapons provide unique gameplay, and so on.
These items, blocks and features are all already available on the network. What will happen to the maps and the game modes? Not to mention the colossal amount of time that was put into reworking the network's code, do they just have to rework the entire thing again according to you? It simply isn't worth it to rework all of that just to include older versions.
Assuming you're speaking about Java: if your device can't handle 1.16.5+, the player's device is literally the lowest of low end... If playing Java isn't an option, CubeCraft is also available on Bedrock.