I think the BFR is just the next logical step. One more size of rocket you can use to do more things. BFR is also simpler, being one big rocket with one landing

I think the BFR is just the next logical step. One more size of rocket you can use to do more things. BFR is also simpler, being one big rocket with one landing to do rather than 3 synchronized rockets with 3 landings. Plus faring size is much bigger on the BFR, which is needed for the Mars plans.