Why Premium Phones don't support Dual SIM cards?

Summarizing for you, few factors plays role in providing multiple SIM support:

• Premium phones need to be lightweight and thinner than ever - exclude multiple trans-receivers required for multiple SIMs.
• Battery backup - Battery packs are thinner and compact than ever, so providing multiple SIMs means beefier batteries and/or less backup!
• Targeted at niche segment of consumers - don't care about cheap deals but cares about lightness/design statement of their phones!

So, the general psychology was that niche customers like their phones to be unique and better than their counterparts - and having single SIM shows the class of the product!

Also some people tells it in different way
Dibya Our Writer Writes about it that

"Let me try to put a more logical answer in the midst of the half jokes here. 

Premium phones try to put a lot in them NFC, WiFi direct, IR ports, waterproofing etc, and they also aim at keeping smaller bezels and thinner footprints. 

An extra GSM radio will prevent doing the above and hence such a decision.

S5 Duos and M8 is proof that companies are realising even premium customers sometimes need two numbers and hence are coming out with dual sim in high end phones."

Give your comment, we shall place it in some next article.