Very simple reason and very obvious one:
- An iMac is much bigger and can hold more ports, thus Apple doesn't mind to include additional standard USB ports.
- A MacBook Pro is slimmer and has less space internally and they decided to get rid of the big USB ports since mobile computers/ laptops are becoming more and more minimal anyway. I'm a pro user and I really don't need all those ports. Most of my stuff happens via the cloud/ streaming and it's not a big deal to use USB-C which is the future anyway. Here I agree with Apple totally and people should stop bitching about changes. It's just like when they got rid of the DVD/ CD drive, now we see nobody uses it anymore. And they were right.
That said I disagree with Apple when it comes to the iPhone. They should've done USB-C there too, which is industry standard. Them not doing so has created a dongle mess and incompatibility with their own laptops.
DVD, CD USB A is still very much alive, maybe just not where you are from and probably a demographic Apple simply does not target, this does not make them right but just ignoring a huge population who probably would never buy a MAC
Those of us who travel with a laptop and not just room to room or school etc know to well crap happens in many situations be it in client offices or hotels. USB-C is just another added inconvenience you have to plan for the next few years and trying to manage using cloud services internationally on the move is just not practicable in many scenarios
Like Apple support that works well in the north Americas and parts of Europe and a few other countries it soon tails off and for many the new MBP are similarly less natively flexible now requiring addons
It's easy to be less critical of Apple when you have same or next day service to hand.
Given the contradiction of ECO system that precludes ports that are the future one has to guess at Apple commitment and intent