megabus.com
does not check luggage or provide receipts for luggage transported on the bus by the passenger. Passengers are advised that megabus.com will accept up to
one piece of luggage per passenger. For guidance this should be
no more than the size of a standard suitcase and should not weigh more than 50 pounds. Passengers can also take on board one small piece of luggage that
must fit either on your lap or under the seat in front of you. Please be advised that passengers wishing to travel with more luggage than one standard sized suitcase and one small piece of luggage that can fit on your lap or under the seat in front of you may have items refused, depending on the availability of space.--
http://us.megabus.com/luggage-allowance.aspx
The reason, I think, is that on the double-decker buses, the cargo bay underneath the floor is removed so the first deck seating is lower. No cargo bay means no room for bikes or luggage. It's not that they're being pricks or anything.
Greyhound still takes bikes and so does Amtrak. I've used both with good results.