As Peter correctly notes, airliners which take off heavily laden with fuel and passengers struggle to reach their maximum altitude capability until they have burned off sufficient fuel. They achieve this by a technique known as a "step climb"

In this technique they keep stepping up from one altitude to a higher one as their performance allows.

Prior to reaching Ukraine airspace MH17 would have flown through rather dense airspace over Europe where they may have had quite strict altitude stipulations, so that even if it had the performance to climb higher that may have been limited by other traffic inbound to Europe flying over the top of them.

As Peter correctly notes, airliners which take off heavily laden with fuel and passengers struggle to reach their maximum altitude capability until they have burned off sufficient fuel. They achieve this by a technique known as a "step climb".

In this technique, they keep stepping up from one altitude to a higher one as their performance allows.

Prior to reaching Ukrainian airspace, MH17 would have flown through rather dense airspace over Europe where they may have had quite strict altitude stipulations, so that even if it had the performance to climb higher that may have been limited by other traffic inbound to Europe flying over the top of them.