Because if it was a case of only a blocked pitot, the plane would have sat trimmed with a nose up bias and very few control inputs would have been required in order for the aircraft to fly away from the ground. It is thought by many ( waiting on report) that the MCAS system trimmed the stab nose down with no command from the pilots to do so, which would necessitate
1/ making more control inputs of greater magnitude in order to allow the aircraft to fly away from the ground
2/ recognising that the trim was running uncommanded
3/ preventing the trim from running uncommanded.
Two different scenarios really.