system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Wars] What were the contributing factors for the Empire to do away with virtually all of the Republic's war machines and create entirely new ones?

An example would be the AT-TE. You have an extremely versatile vehicle with more uses than just combat or artillery, that can provide secure transport and act as a Mobile Operations Base. Why would they scrap these formidable tanks when they could still be put to use outside of warfare?

A conversation with a co-worker a few years back introduced me to the idea that the Empire simply didn't have any need for heavy duty combat vehicles, especially while fighting the fairly outgunned Rebel Alliance. He said that the Empire would focus more on anti-guerrilla warfare with vehicles like the AT-ST which were highly mobile and heavily armed. He also said that battles like Both show that the Empire only needed a fractional force to crush a Rebel base, and that high mobility and visibility was the key to snuffing out insurgent emplacements and fortifications.

I feel that while the development for these kinds of vehicles were certainly a well thought out plan, the abandonment of certain vehicles that performed well even outside of combat was unnecessary.
gpt
There's a handful of reasons that crop up over the lifetime of the empire. 0bby to around 14bby when the largest changes occur is because of the clone army being removed. The grand army of the republic was restructured rapidly into the imperial navy and imperial armed forces. These needed a vastly larger industrial base and long term viability in their technology. They weren't needed to wage wars but police planets and space lanes and project order and stability. The clone wars were an excellent testing grounds of future imperial designs but ultimately too narrow and expensive for long term results. Cheaper star fighters that could out maneuver nearly every fighter and small vessel became more important than heavy bombers and multi-crew multi-role fighters. Star destroyers improved on previous designs to act as mobile fighter bases and force projection over smaller more niche frigates and destroyers. 
Palpatine also was rebranding his empire. Anything associated with the republic from the iconography on buildings to the names of bases and the tools were thrown away and made new to distance the empire from it's origin. 

4bby to 5aby it falls under the tarkin doctrine. Fear and power become design features as well as cost effective designs. Everything was both cheaply mass produced and designed to crush both physically and spiritually. Weapons were built to demoralize the enemies of the empire through just how devastating they were when deployed and how numerous they were. A single star destroyer brought an army a fleet of fighters and bombers and enough firepower to level any resistance. And the empire rarely sent a single star destroyer to a problem that could be solved by one alone because they could afford the excessive force. Enemies of the empire were drowned in blood and metal and blasters if they didn't surrender immediately seeing the forces that carried the Empire's banner once they arrived.