Upcasting is easier to code (I would assume) then metamagic would be. Because we know upfront what a spell's upcasted version is. Most of the times more damage, more projectiles or whatnot. Metamagic can be more nuanced then that though.
You can turn spells that cost 1 action to cast into bonus actions.
Make spells that only target 1 target suddenly target several.
Increase range, or in the case of spells with no range; give them range.
etc. etc.
Its alot more then just making upcast versions of excisting spells. Also if they would turn it into modifiers, when would they apply? Before casting? Would activating the metamagic feat that you want to use swap out your spells for different ones? If you want to remove somatic or verbal components, how do you pick between the 2 and how does it effect the spells? Is it easy code to just remove that portion of the spell in that case? Not to mention that if you got a metamagic option, it has to work for all spells that you can have. Add multiclassing to the mix and thats alot of spells that they have to make sure work properly.
Having said that, I dont code. So for all I know its actually alot easier then I think it is. But I can inmagine that with the big number of spell and metamagic combinations possible they needed some time for this. If nothing else, they got alot of variables to deal with. Because of that saying that Sorcerers are easy to implement isent necesarrily true I feel. Theres alot of things that metamagic can influence when you factor in dual classes. I plan to play all the casters with a single level of sorcerer just to see the wild magic shenanigans
I would try to test the way metamagic works with other classes but uh...... Lvl 3 sorcerer/lvl 1 whatever class doesent really allow that much testing I think
Also, bear in mind that most of the classes have 1 or more subclasses that gain spells from excisting spellcasters. Even the barbarian can gain some spells that he can cast. It makes sense to get the spellcasting done first, so if you need those subclasses' their spells (or feats that handle like spells) you have them on hand. Aside from that, most of the martial classes are pretty....simple. I think they want to get the more complicated things done first before they do the simpler ones to round up the classes.
