This is undoubtedly the #1 question/suggestion we get in letters from readers, reader surveys, etc., which is pretty gratifying. And after all, who doesn't want more of a good thing? There are several reasons why FSM has stayed at 10 issues/year for a while.
First is advertising revenue. Not all our advertisers place ads in every issue. Adding two more issues will not necessarily result in 1/6 more ad revenue. Second is cost: we'd need to add more editorial and art staff to handle the additional work and to work on getting more stories to fill the additional issues. (If advertising revenue doesn't cover this cost, the only other significant source would be raising the subscription price--NOT a popular idea!)
Third is the general state of the modeling hobby industry. Many of our feature stories don't necessarily depend on the availability of new kits--scratchbuilds, conversions of old kits, demos of techniques, etc.--BUT each issue of FSM has regular departments like Workbench Reviews and News & Products, which are dependent on the rate at which the manufacturers issue new products. With the industry not growing (but weathering the current recession better than other industries--compare it with, say, the recording industry), we're cautious about committing FSM to delivering more information than we're getting in.
I won't say that FineScale will never go monthly, but I doubt if it would happen before a substantial recovery of the general economy gets underway.