Previously, $1.000.000 spent on R&D in 2023 would result in a $1.000.000 deduction on your 2023 taxes. Under the new system the same spending would result in a $200.000 deduction in 2023, $200.000 in 2024, $200.000 in 2025, $200.000 in 2026, and $200.000 in 2027.
You still get the same deduction, but spread out over multiple years. However, it also means that you can now deduct $800.000 less in 2023 than expected, resulting in a far higher tax bill this year! If you are a startup you probably don't have that spare $800.000 just lying around doing nothing.