Because if it was a case of splitting his forces to several locations, he wouldn't have waited until the latest development.
Having said that, what makes the most sense to me is WCI; which I initially mentioned when the chapter was released at the time. Attacking Marie Geoise doesn't make sense to me, as the Reverie is over and so thing are back to normal and he'd have to pass Marine HQ (with its high ranking officers back in position).
So we know that whatever BB is going after:
- He had prior knowledge of and now circumstance have allowed him to finally claim it.
- The WG/Marines are also after it.
- It involves a piece of news revealed in the newspapers after the Reverie was over. Some things that were revealed in the papers:
- An incident involving Sabo and co to take back Kuma
- The disbanding of the Shichibukai
- The Supernova gathering in Wano and Big Mom chasing after Luffy (ch925)
- An incident involving Alabasta
Though Big Mom's actions were revealed in ch925 and Teach confirms his move in ch956, I think the latter is using Big Mom's travel to Wano as an opportunity to move into her territory; be it for the Poneglyph or conquering her kingdoms...or both. Her forming an alliance with Kaido doesn't seem to have made the newspapers yet, the Marines being aware of it because these two spoke without using white den den mushi.
The way I see it, a possible outcome is Blackbeard moving to WCI, which will force BM to be recalled to WCI with her forces before she becomes too involved in the current fight. Leaving Kaido to deal with the alliance with his and Orochi's forces.