Obviously because of the geopolitical concerns and pride. Truth is the first victim as usual. The Russian vaccine was indeed registered just after the 2nd phase trials, but according to its website the 3rd phase trial has started on August 25 (one month later than for BNT162b2) and involved more than 40k people (effectively the same number).

So those who claim that Russian vaccine is only on the 1/2 phase spread an outright lie. Both vaccine have approximately the same speed of development with minor differences between regulation and testing procedures.