Map Men definitely took off. Over 1M per video for the past few years. They (he?) just don't make enough videos to grow as fast as Tom's channel. YouTube rewards consistency A LOT.
Yes, I'm sure that's a factor too. Also, one of my first reactions was, unless you have an explicitly comedy kind of channel, maybe somehow audiences don't seem to reward British-style quirky humor that Map Men is full of. I love it, but maybe it causes people to feel it's not "professional" or professor-ly enough? Tom Scott definitely doesn't do humor, at most only wry ironic suggestions. The Map Men sponsorship endings themselves have been hilarious (Surfshark, NordVPN, Audible, etc).