Because the BBC Micro was comparitively expensive when up against the likes of the Oric/Spectrum/C64 etc Acorn redesigned it, took away nearly all of the expansion capabilities, tube, floppy/printer/user connectors, econet etc and lobbed all the ROM code and supporting chippery into a single large ULA to get costs down and released it as the Electron, largely to people saying 'ooh its a cut-down beeb.' The name was consistent with their other naming conventions - Atom, Proton (aka beebon, aka BBC Micro), Electron.

Even the BASIC was cut down so a lot of BBC BASIC programs wouldn't work either a) without modification or b) at all. Looking at my Electron software collection there seems to be some ported BBC stuff but mostly it's conversions from other platforms. I had a go on Paperboy the other day and I was surprised at how Spectrum-like it was!

Expansion wise nothing was available until the Plus-1 was released, which provided a printer interface and a couple of ROM slots amongst other things. Companies like Cumana produced a ROM cartridge to allow you to plug in a standard Shugart floppy drive, then Acorn themselves released the Plus-3, which provided you with a 3.5" floppy drive and a through port so you could have the plus-1 and plus-3 on at the same time, though it has to be said that once you had that lot on you had one big mother of a machine :o) This could be why the Plus-1 has no through port.

"what happened to the plus-2?" I hear you all think. Dunno!

These days there's approximately no enthusiasm for these machines any more, this isn't helped by the fact the ULAs could cook themselves, and obviously the only way you're going to get another one is from another Electron.