Certain personalities in Malaysia are sacrosanct - Samy Vellu has been in politics long enough to know that.
Even the previous Prime Minister Tun Abdullah Ahmad Badawi found it hard to criticise Mahathir without having to deal with the repercussions.
Mahathir had been in power for so long that an entire generation of Malaysians grew up during his reign. These younger Malaysians have known no other Prime Minister (okay, Abdullah ruled for a couple of years and now Datuk Seri Najib is in control), but what I mean is that no other personality has left such a deep mark on them. Abdullah's tenure was sadly quite forgettable and Najib has just started things rolling.
Mahathir is that kind of historical figure who achieved greatness. Now that must be something way beyond the reach of Samy Vellu...