Because new technology or maybe feature inside AF-"P". AF-S still compatible in d7000. I already try this afp 70-300mm into my D3100, it can't snap or take even 1 pic.
If you own one of the following cameras it is fully compatible with AF-P lenses: Nikon D500, D3300, D3400, D3500, D5500, D5600, D7500. Please note the following cameras are not compatible with AF-P Lenses: D3000, D3100, D3200, D5000, D5100, D5200, D5300, D7000, D7100, D7200, D40/D40x, D60, D70/D70s, D80, D90, D100, D200, D300/D300s, D600, D610, D700, D750, D800/D800E, D810/D8 D850 from Google