I don't think Palpatine wanted anything to do with the CIS. He wanted to start completely over with the Empire.

You're English was good, don't worry!

The CIS were a means to an end, and they weren't meant to do much other than be enough of a nuisance to threaten the Republic enough to become the Empire

As well, most CIS ships didn't fit very well with what would become standard Imperial fleet doctrine: massive battleships capable of theoretically threatening anywhere in an entire sector on their own, no assistance required

This also applied to the Venator, which... didn't really have enough of a presence on its own to fill that role, especially not with TIE fighters (which they'd have to be adapted for anyway)

2 answers to that

1. Tech advances with time. By the time of the empire's golden years they may have been advances in technology, and overall it was probably easier, quicker and cheaper to build new ships around the new internal systems that to go through the old ships upgrading each part.

2. The star destroyers are a symbol of the empires power. The emperor didn't want ships that look like a throwback from the previous era- he wanted something strikingly new, not a symbol of the power of an old enemy.

Because of the Tarkin doctrine the Empire created it’s own identity and military equipment.