Are Tau even near Ultramar?
Do they even have the resource?
Their schtick is that they're a relatively tiny faction. A fleet(not even the main one) of Death Guard decimated their Navy. DG retreated because they took too much damage before they could even start landing on the planets. The Tau won a Phyric victory after going all out against just a fraction of one Chaos legion

I find it funny that Tau are flanderized as peace hippies and most of their fan art is busty blue women
Reading their stories and campaign books they're in they actually come across as pretty warlike and aggressive who put a premium on honor, they even do honor duels and trial by combat. They honestly come across more like Klingons and Mandalorians.
But when your competition are World Eaters and Orks they don't seem that aggro.