The 22-year-old was set to join the Hammers on a season-long loan but decided against the move just minutes before the window closed.
Speaking on the Chelsea Mike’d Up podcast, Tomori said: ‘I had a plan going into this season going off the back of last season.
‘Obviously, in football, things change so quickly so I wanted to make sure whatever decision I made was really thought out and to try and do what’s best for me.
‘I made the decision in the end to stay, to improve more, get some more games, gain some more experience and try and get to the levels I was at last season.
‘I wanted everything to be thought out and I thought about what was happening, especially when we had a few weeks off leading up to the new season.
‘I was talking a lot with my agent, to get some clarity what was going on. With transfers I prefer if it wasn’t last minute, but situations happen where you have to take a decision quickly.
‘It was one of those situations and, as you all know, I made a decision to stay.’
Tomori has made three appearances for Chelsea so far this season and could be involved on Saturday when Southampton travel to Stamford Bridge.