There have been many good explanations to this topic already that are quite valid. I have two other possibilities as well.
1. The Enterprise had been relegated to the role of a training vessel and quite possibly wasn't up to the level of other ships on active patrol or exploratory missions. It may have been some time since it had last been overhauled or had major improvements to weapons and power systems. The primarily trainee crew wasn't as experienced and the casualties suffered limited their ability to make field repairs.
2. The Reliant, I always felt, was a ship that had been designed as a direct result of the Constitution class and it's successes, while seeing improvements in certain areas. It may not have had as powerful a warp core, for instance, or inferior shields and perhaps thicker armor to compensate. It obviously wasn't designed as a deep space explorer vessel and probably not intended to operate at such long ranges and alone for extended periods. This probably evolved into it being armed differently and the adding of the rollbar type attachments to make it more flexible in local missions or to complement armament, sensors, etc., as needed.
Here are some alternative possibilities:
1: The Enterprise in the 2280's was re-designated as a cadet training vessel while the old Republic
was pulled out of service temporarily to be rebuilt to TMP-era specs.
2: The Reliant, Saratoga, et al are all starships-of-the-line, but they are not cruisers. Perhaps they are the TMP era's take on what FJ was trying to do
when he came up with the Hermes/Saladin Class I destroyer/scout type
vessels. Maybe the difference between a cruiser and a frigate/destroyer/scout/whatever-you-want-to-call-it is that cruisers have more overall mass, greater crew and cargo capacity, and separate, distinctive secondary hulls. Reliant obviously lacks these traits, so she may not have been a cruiser.