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What is the technical reason why bitcoin transactions can't scale?
Because they cannot be joined. There are many transactions like that: Alice -> Bob -> Charlie -> Daniel. That could be replaced with Alice -> Daniel transaction.Quote
why we are not seeing many changes to bitcoin
Because every big change have to be done in a soft fork way, discussed and developed for months, and later activated by miners signalling their readiness for that change.Quote
why the 10min transaction times is something which "must" occur in order to maintain security
Because the network is more decentralized if this time is not too short. If you have shorter block time, then when you hit some block, you can build instantly on top of it. The shorter that time, the more blocks in advance you can create, turning lottery-based mining into race-based mining when the more power you have, the more boost you get, which is unfair.Quote
With the internet, connection speeds increased from 56kbps dialup to adsl to cable to adsl2 to now 500mbps. We could see progress being made and changes to the technology.
The biggest bottleneck is not downloading time, you can download all blocks quite quickly. The verification is what takes the most of the time. If you take some altcoins, especially CPU-based with slow and complicated mining algorithms on purpose, you can clearly see that. But in Bitcoin, ECDSA verification also takes a lot of time. And when it comes to processing speed, we stopped at around 8 GHz and going faster is very hard. 
