Why was this though? Why can't a language on a modern computer have a similar learning curve? Or even, why can't BBC BASIC be ported to a modern computer so we can have a just as productive language with a low learning curve?

Was it because the hardware and operating system were less complex? Well, the raspberry pi runs Ubuntu on an ARM processor, so its not going to help in this respect.

I suspect it seemed simpler out of necessity - if you didn't learn it, you couldn't do much with the computer, while nowadays you don't need to know how to program to get a lot of value from a computer. I've read countless stories about how kids learnt BASIC and then learned the systems assembly language because BASIC was too limited or slow. The core instruction sets of todays processors (eg x86 or ARM without extensions like FPU or SIMD instructions) aren't much more complex[1], yet nowadays you rarely hear of anyone using assembly, let alone kids. Its not that learning assembly is harder now than it was, because its not harder (if you ignore the advanced instructions - which you wouldn't learn till later anyway) because the core instruction sets are still very similar, there are better tools and editors available now, there are emulators and simulators and there is a whole internet of tutorials and articles, while back then you probably only had a single manual. The difference is that today you do not need to learn assembly to do cool things, while back then you were pretty much forced to once you outgrew what the BASIC system could offer.

So I'll ask again - what is it about the programming systems available at the time that makes them easier to learn than those today? Besides the fact that learning was a necessity (and one which the Raspberry Pi cannot realy hope to emulate, IMHO).

[1] Well, x86 is a mess of an assembly language compared to others (I've been doing a lot of PIC24H microcontroller assembly programming lately and its much nicer to work in), but its not exactly complex compared to other (including the older ones I'm comparing to above) assembly languages.