The problem with relying on short range weapons is that before your ships can even get off a single shot the long range weapons of the other fleet have carved up a good portion of your ships. Even when you close in, lets say your ships have 1 enemy ship in range. Thats all they can shoot at, and then they need to move again.
Well, the other fleet with long range weapons can fire at all of your ships. This means they can far more easily focus fire. I have yet to see any cruiser withstand more than a few seconds of focused plasma from a dozen or more cruisers. Shields drop and then the beams carve up the hull. A shieldless cruiser being hit by 20+ beam weapons and 20+ plasma launchers doesn’t last a very long time.
Before the CL fleet can even get off a single shot the long range fleet will be taking a huge toll on their ships. As each ship comes into range it is focused fired down by all of the long range ships, one after the other. Yes, eventually they will get through due to sheer bulk of ships, but by then they usually don’t have enough firepower left to make much of a difference. You’re basically sacrificing over half of your fleet merely to close to firing range. Meanwhile you have yet to do any damage at all to the enemy. Even if the remaining half of your fleet is victorious, you’re still going to take massive losses, such that you probably will not be able to sustain that sort of fleet in any campaign situation.