Probably many reasons. Similar to the Vietnamese encoding wars before the widespread adoption of Unicode.
And i suspect that lack of wide spread font rendering/text layout solutions is part of that
But the prevalent use of that font in web pages and blogs is another factor
But as Jimmy commented on about communities building up, I suspect that Zawgyi has had such a similar phenomenon, it is a mechanism of display etxt in various social networks that have evolved on the internet. So people becoem atatched to it.
But proper support of Burmese and minority langauges do call for Unicode 5.1 compliant solutions both short and long term.
As Michael indicates it is important to have solutions in place for Sanskirt and minority langauges as well as Burmese on
my.wikipedia.org. To his list i'd also add Pali.
I find myself in a difficult position, working for a state library, a SA. I have to support licesing and copyright. Wether its open source, creative commons, a commercial EULA. And Myanmar Unicode 5.1 solutions are relatively hampered in the Windows environment by licensing issues.
Being involved with W3C inetrnationalisation activities, I strongly support web standards and best practice, esp with respect to internationalisation and accessibility (in its broadest sense) and from that perspective Unicode 5.1 is the best approach.
I'd agrue their should be a single community building articles for wikipedia in Burmese. perosnally transcoding for end users is an posisble interim measure. Although I'd suggets that editors are encouraged and aided in migrating to Unicode 5.1, and that transcoding is left to supporting thaose end users that require it.
Michael and others, I'd like to assist in analysing the fonts and CSS if there is anyway I could help.