It's inevitable that showmasters will eventually put on a horror only con. For the last few years I've understood their argument that they won't get the same numbers that say collectormania bring in... but things have changed over the last 2 years.
Helped by the resurgence of vinyl and now VHS tapes, the horror genre is now the most popular it's been since the mid 80s. Just think about all the new horror films that are being released lately. Also, there's so many new companies being born to accommodate the need for new horror memorabilia.
Off the top of my head I can count 6 horror cons the UK is getting this year. 2 years ago, there were none!