What, apart from ego could be the basis of this statement?
There is different vehicles because there is different level of practioners, due to the two accumulations some are nearer buddhahood than others...
Ok, if you ask for explanation, it's better ... there it is
Beings progress, from one life to another, by accumulating the two merits. Therefore, in the same given historical time, there is beings which are not yet on the (dharma) path, some are on that path since few lifes, others are on that path since many, many rebirths, accumulating merits. All those beings, on the way to buddhahood, are following teachings and are practicing the dharma. Some are following the path of renunciation, the sutrayana (4NT), where the practice is based on the material (physical) level, it is said that once on that path it would need countless rebirth to attain buddhahood. Some others are following the path of transformation, vajrayana (tantrayana, mantrayana), where the practice is based on the energy level, it is said (depending on the tantras followed, external, internal) that once on that path it would need some rebirth to attain buddhahood. Again others, following the path of self liberation, dzogchen, where the practice is based on the mental level (mind, nature of mind), it is said that once on that path, buddhahood could be attained in this life or during the bardo after this life. This is (briefly) the long path of the dharma of Buddha Sakyamuni.
The reason why one is on that or that yana on that path is therefore related to the merits accumulated during numerous previous life.
What one has to considere is that, wether one is on sutrayana or one on dzogchen, there is no reason to be pride (or ashamed) for that, because accumulation have been made by "other beings" in previous lifes. One just inherit of such or such accumulation (and consequences) and therefore there is no reason to be proud if ever you are following (for exemple) dzogchen teachings of such master, because first it would be counterproductive in term of buddhahood, but stupid because what count is not the karma you inherited but what you will do with that karma during your present life. And for the same reason to be ashamed is equally stupid ... there is a point where a practitioner does not see anymore in term of pride or equally of shame.