We are not imperial guard fighting symmetrical enemy in some plain field with big enough open spaces for usage of vehicles like leman russ and good luck getting more rare specialised versions of leman russ for urban combat, and even then tank is worthless without established infrastructure and experienced crew.
With genestealer cult as enemy, we face rapid breeding stealthy subversive apex predators, who have no problems with suddenly picking slow tanks with inexperienced cultists inside by adamantine claws in dense streets of hive city.
Legionary : comes with own equipment( which was designed and made by brightest minds of adeptus mechanicus during golden age of mankind, and surpasses anything your average manufactorum can make ) , is quite likely veteran of long war with centuries or even milleniums of combat experience( and is quite adept in providing said experience to cultists for destroying rotting imperium)
is geneforged to be perfect machine for slaughtering xenos and knowing no fear,
despite being walking tank has no mobility problems of tank( you cant fit leman russ in mines or sewers and tanks are not designed for urban warfare in general)
Also tanks riding around hive streets or guard tier weaponry usage is more likely to attract HAMMER OF SOME ORDO on our heads, unlike some of best stealth warfare specialists of galaxy.
I would suggest attempting to focus more then one dice on alpha legion gambit, so
we dont get dunked by 1 bad dice roll