Only incentives I can think of:

1. They didn’t police it properly for the size of event and it’s a pre-emptive defence. “Yes we only sent one constable, but it was only 3 old ladies and a Yorkshire terrier”.
2. They’re under orders to downplay the size of protests that can be seen as anti-government.

I know it’s a bit tin-foil hat, but I’m starting to suspect the latter. A number of protests appear to have had quite large numbers from footage (which is likely to be cherry picked yes, but should set a minimum number), but small number from the Met.