While important, the arrival of an ambulance on the scene of an accident can probably usually wait for a few more minutes than the arrival of a fire truck.
If there's no rescue crew there on time, then the need for a medical team is likely to be moot.
The majority of the emergency response group did not appear to arrive on site until more than 20 minutes after the crash - as can be seen in the amateur video of the incident. This is backed up by the media reports that passengers who escaped and made their way towards the approach end of the runway, who found and were attempting to care for the seriously injured passengers, had to call 911 more than 20 minutes after the crash and essentially beg for help.
By all appearances - even 20+ minutes after the crash - there was almost no no emergency response to much of the accident scene. In the video you can see a police car race past the fuselage heading towards the approach end at high speed - but if memory serves me, that was just slightly before the larger group of ambulances etc arrived there.