In truth I don’t think many came after Warcraft 3, but The Frozen Throne expansion would come the following year and WC3 would maintain a pretty impressive player base over the next 10 years.
Sins of a Solar Empire released in 2008, but by that time the RTS genre had fizzled out and most entries would fit more into 4X or Grand Strategy.
If this was the reason then they’d be trying to sell us these old games again…but they’re not outside of their shitty subscription service that offers a few games that nobody wants.