My question is why couldn't they maintain altitude
They never managed to attain the V2 of 220kt
td, highest speed reached 211kt, not helped by having to avoid the holding 747 which they flew over missing the 747 by a matter of feet according to the cockpit crew. Last airspeed recorded was 136kt immediately prior to the crash.
For the weight they were at the zero rate of climb speeds were,
Gear Retracted - 0ne engine out 193kt - Two engines out 262kt
Gear Extended (the condition they were in) - One engine out 205kt - Two engines out >300kt
They had two engines effectively out.