They have organised almost allforms of music genres there but why not metal/rock.
If I may throw out a guess... Because nobody has been able to convince them to do it yet? Besides, I don't think they have organised 'almost all forms of music genres', but perhaps most of the 'most popular/mainstream music genres'?
Maybe it helps if you think of the Esplanade's positioning as a centre of the arts. That said, there may be certain forms of arts (loosely used) which they probably feel do not align with their (and their sponsors') corporate/public image and values. Also, they will definitely have their criteria on what qualifies as 'art' in their books, and what does not. Whatever those may be, *shrug*
Example: It doesn't seem quite likely for the Esplanade to hold an exhibition on body modification just yet. It will be more likely to find an independent gallery keen/willing to host the exhibition.
Maybe that's the type of profile the gallery wants to raise - fringe, experimental, bold, etc. Maybe the people who give money to support the gallery are not stiff-necked corporate suits who want a place to stand around and sip champagne, maybe they are experimental artists themselves and can identify with the values this particular exhibition brings.
Which ultimately means that this is the type of gallery that a Chinese calligrapher will not even think of approaching for his exhibition - he'll probably head to some place like NAFA.
Anyway, I do feel that it's not something written in stone, as long as there are people like you, ShyamRaj, passionate about what they do. As you lead the rally, maybe you can remind the Esplande that one of their Values, as listed on their website, states:
Take ownership and risk
We take pride and ownership in what we do and will lead by example. We will constantly seek new ideas and have the courage to take risks. In pushing our limits, we will accept that failures are part of the learning process.
http://www.esplanade.com/SOPApp/espsop/portal_proxy?uri=ddz-m5lf!y74mwl0t.UJ9=1QhhLO3YA7mnL6sNI4.N@5sJFM
They're smart though, right below that, they state:
Be prudent with our resources
Our resources are scarce and precious. We will always be responsible with them, exercising financial prudence and leveraging every opportunity to stretch every dollar.
Some random thoughts of mine.