But running a SC does have overhead and risk. It seems obvious to me that companies will use the mainchain if possible (that is, it scales and can capture the transaction).
There are significant number of people who don't want the main chain to scale, for various reason.
Sidechains provide those people both with an argument against scaling the main chain, and it creates a group of people with a financial interest in preventing the main chain from adopting new features which might render particular sidechains less necessary.
The latter part wouldn't be such a big deal if Blockstream wasn't so closely connected with Bitcoin Core development.
the SC's idea was hatched in the minds of the Blockstream ppl at least a year ago from an interview,iirc, and someone put up a link on bitcointalk from a thread by killerstorm initially discussing the idea maybe 2 yrs ago. they then filed the trademark in April 2014. so we know they've been thinking about making this move for at least a year.