Because the majority of people looking at the topic correlate the introduction of any sort of central bank digital currency with the introduction of efforts to curb the use of physical cash. India in particular had a recent change in their legal tender ( https://en.wikipedia.org/wiki/2016_Indian_banknote_demonetis... ) which has made some people concerned about these sorts of moves. If you’re willing to suddenly (from the perspective of those aggrieved by the changes ) declare a significant percentage, over 75%, of the banknotes in circulation as soon to be defunct and basically “turn over” your entire currency over like a compost pile in under a year, people get understandably concerned you might just declare cash illegal entirely as the government has established itself as willing to do dramatic things.

Because it allows them to enforce policies on where and how you spend your money. In an authoritarian regime this can be abused pretty badly.

It's different like this:

"Citizen, too much meat eating this week. Your CBDC non-cash is now suspended until you lose 10 pounds"

A cursory google search on your part would have easily turned up web pages like these from the central bank of Canada:

https://www.bankofcanada.ca/2020/02/contingency-planning-cen...

https://www.bankofcanada.ca/research/digital-currencies-and-...

CBDC are just a huge/national bank. It's a dream come true for the Marxists.