So I was looking through the Incredible Cross Sections book and Imperator-Class Star Destroyers have two side hangars each with two openings; one the outside and one to that big space where the Tantive IV was taken into in ANH, but Venator Star Destroyers have extremely similar hangars to the Imperator-Class with a underside opening and two side hangars but it also has a very large front hangar section with many hangars lining the bow of the ship and then the bow opens up allowing those ships to take off. Why did the Imperator-Class remove the bow hangars and just keep the side ones? Is there a reason for this? I want a Legends answer but I will take a Canon answer as a secondary answer.
The larger hangar was unnecessary.
According to various Cross-Sections and other reference works, the standard fighter complement of a Venator was up to 420 fighter craft (192 V-wings, 192 Actises, and 36 ARC-170s), plus either 20 or 40 LAAT/i gunships (depending on the source) and some unspecified shuttles.
In contrast, the Imperator has various configurations of TIE fighters but always 72, plus perhaps a few dozen shuttles and landing craft of different types.
Given that the total number of small craft was cut severely, it's not really a big surprise that the hangar space and launching capacity was reduced as well.
The bow hangar was a weak point.
The large bow opening allows the Venator to launch a lot of fighters at once, but it's also a very large piece of the ship to be moving. It was slow to open and close, and was relatively easy for an enemy to attack. This might be a worthwhile tradeoff when you have 400 fighters to get into space all at once, but less so with the reduced fighter complement of the Imperial Star Destroyer.