"Why if you take bonus it gives the whole amount but if you take regular it figures it at a fraction of the year?"

That's why it's called Bonus. Otherwise, it's regular.

"If a client puts a new car into service on 12/27/20"

That means 4 days of business use for the year. You put it into service late in the year, so you don't get much depreciation that first year.

"Is there any reason why we would not want to take bonus depreciation?"

Yes, that's part of tax planning and business management strategy.

It seems you have some reading to do:

https://www.investopedia.com/terms/b/bonusdepreciation.asp

https://www.irs.gov/newsroom/new-rules-and-limitations-for-depreciation-and-expensing-under-the-tax-...

https://www.journalofaccountancy.com/news/2020/jul/2020-irs-inflation-adjustments-cars-trucks.html

(click their links to see IRS Rev Proc).

https://bradyware.com/business-vehicle-deductions-2020/

google:

why take bonus depreciation on vehicle