They most likely just learned they can simply keep TS4 on life support with drip fed content for years and still make money. Compare the difference in amount of content from TS3 EPs, for example. Hell, or simply just note the ever so amusing fact that TS3 still has more content than TS4 despite being alive for 2 years less!
I mean, why bother making something new aka spending money and actually releasing us from the suffering that is TS4's awfully limited engine born out of an always online fiasco until they absolutely have to do so, especially when it's still profitable for them to keep things this way. Mind you, I'm pretty sure TS5 is already in development, after all, there's only so long they can keep TS4 alive.