Why did it take so long for mobile electronics to develop?

I've noticed a trend
In the last 20 years, battery powered mobile computers have somehow reached, and overcome a climax. Since the 1980s, handheld devices have existed, but only in the last 10 years have they become powerful enough to be viable as a general purpose computer.
In the first half of the 21st century, e.g pre 2005,
The handheld devices we had existed in a decade before the one we were in. Which was the 1990s.
Before 2005, the average PDA, cell phone or palmtop computer had less processing power than a 386 desktop. Most of them had less than 4 megabytes of RAM. It wasn't until the second half of the 21st century, eg after 2005, that Pentium class hardware became optional for handheld computers.
in 2006, Sony Computer Corporation developed the PlayStation Portable. This was the first mobile device to utilize a true 64 bit microprocessor, the MIPS R4000. The r4k CPU in the playstation handheld was the FIRST computer since the Intel P5 Pentium pro to code and decode using 64 bit arithmetic. It was the FIRST mobile CPU to outperform a Pentium.

It wasn't long after that the things began to pickup pace. Along came the iPhone, and then Androids followed. In just a short few years, mobile technology has gone into hyperdrive. Now, in that short period of time, it isn't uncommon to see advertisement for smart phones that have MORE memory and more conputing power than the finest damned desktop PCs only 2 years ago.
what happened?

Computers double in power/speed/overall performance, every 5 years or so. Maybe less now. So every 5 years it got better, but no one was mass producing them, so no one really noticed. Then it became viable. Everyone saw that it was going to make a lot of money, and then everyone started trying to make portable computers. When the human race sets its mind to something, ooh boy do we take leaps and bounds.

Size was the issue, and the power versus performance ratio. Transistors weren't even developed until the early 70s, and those took a TON of juice at the time.