Actually it isn't, that would be Japan and Hong Kong. PH is probably 3rd. What you see though is that PH features prominently on eBay as a source of vintage seikos, while Japan and HK don't. Do look at Japan's Yahoo Auctions though, and you will very quickly see find where seiko central really is - especially if you are looking to collect really old seikos.
The philipines, was a major seiko market for many decades, and seikos were the dominant brand within it. That is not so much the case any more. the watches were popular because they were inexpensive, came in many styles, they were solidly built, and were easy and cheap to repair/maintain. Unlike markets like the US where old watches are considered disposable, in the Philipines, these old watches were often continuously worn for decades, or repaired and resold. From what I understand, many of the international eBay sellers located in Thailand and Singapore who specialize on vintage seikos, tend to source much of their stock from the phillipines as well.
another factor is while the Philippines is now economically the sick man of asia we were for a very long time the most economically capable country after japans surge in economic activity. We were the biggest consumer in asia. Add the US bases to that and you have what youve affectionately called seiko central.