Why does everyone call the old UCP uniform ACU’s? When the current OCP’s we wear now are ACU’s ?

Because the Army Combat Uniform (ACU) was the replacement for the Battle Dress Uniform (BDU), and the UCP camouflage was the first type of camo used for it. So UCP and ACU became sort of blended together, because there was only UCP for the ACU. It's similar to how BDU's (And I've heard ACU's) were sometimes called "Cammies".

The UCP first started being issued/sold in 2005, and OCP didn't start rolling out until 2010-ish for deployed personnel only (Correct me if I'm wrong), and started being issued to trainees in late 2015. That's 10-ish years of "ACU = UCP" for the main force and it just stuck.

Regarding your "If I mentioned putting on ACU's they think it's the old pattern", that shouldn't really apply since OCP has been mandatory-wear since October 2019, unless your unit is just using old gear (Some people in my unit still go to the range in UCP armor and accessories).

The uniform name hasn't changed. Only the pattern. Both sets are the Army Combat Uniform.

Because they don't know any better. ACU is a style of uniform, UCP, OCP is a cammo pattern.