It was probably kept below 6000 ft by ATC restriction due to inbound traffic from the south and south east of Singapore. Quite normal when runway 02 is in use at WSSS and there is a lot of traffic inbound. Once clear of Tanjung Pinang (Tango India NDB/TPG VOR), it's usually unrestricted climb from there to cruising level.
Ok that is most likely the reason. Thanks wsss