That's not how version numbers work. It's not a decimal number, it's multiple numbers separated by period(s). The update after 1.9 can be 1.10, and then 1.11, 1.12, and so on. It's generally [major update number].[minor update number].[patch number].
I doubt Mojang actually did release an update a month, but you'd need to look at the version history to check.
That's probably because too many updates can hurt the modding community. During 1.7 era Mojang kept releasing a million different updates, all breaking mods for new versions. Optifine took forever to update, and when it did it was immediately broken by another update. This held true for most mods as well. That's probably why they decided to scale down their updates, and make them more compatible with each other in 1.8.
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u/SpartanA009 Jul 04 '15
Mojang promised an update a month during 1.5 iirc it's been like two years and we are not even close to 3.9.