From what I am told there are basic compilers available for other micro controllers.
AFAIK the primary thing slowing picaxe is the fact that each chip has been flashed with a bootstrap program. when you compile your picaxe programme and upload it, it compiles to a list of "tokens" which is read by the bootstrap programme, Each of these tokens takes a few extra cycles to read over the fully compiled software. As a side note, most arduinos run @16mhz, but will often match a picaxe running up to four time the clock frequency
Although I don't know much about it, arduino use a bootloader to allow easy programming, this can be over writen (if you need more code space), or writen to a blank atmel chip. This is perhaps another perk that picaxe has missed (although i understand why). The bootloader does take a little space on the controller, however the code that you write is compiled down to the instruction sets for each of the controllers that you can use, this is why simple plugin can be used to allow for other controllers to be programmed by the same software.
All of that open-ness can cause revenue stream problems, the software is free, plugins are free, librarys are free. All you have to pay for is the boards, and again due to being open source there is an obundence of 3rd party boardswhich I am sure takes from the revenue stream. But being open source pretty much garrantees that people will make things work and publish there work meaning you ewnd up with a large and healthy community to support the system - and also enough so some will buy genuine boards to keep the ship a float, so to speak. Its a risky business model but they seem to have made work for them.