That's a very good question. If we believe an old FCOM, the lever has to be recycled to get the gear down below 260kts. The valve won't open if the lever just stays down when the speed goes below 260kts.
The FCOM isn't too clear on this, and of course we don't have access to the manual for that particular aircraft. Current info is that "Below 260 kt, the hydraulic pressure supply remains cut off as long as the landing gear lever is up." I can't find any reference to having to recycle the lever in the PRO or SYS sections in recent FCOMs, a system chart shows that an indication of speed < 260 kt from ADR 1 or 3 and the lever selected down should open the safety valve and allow gear extension. The procedure may call for recycling the handle but I doubt they had time to refer to the QRH.
If they were 5nm out and going so fast that the landing gear couldn't extend it's unbelievable that they continued, even if they had been on the correct vertical profile they were around 100kts too fast.
With the recorders available and being read by French investigators, together with the marks on the runway we should have an accurate sequence of events fairly soon and won't have to speculate on what happened. We can instead look at WHY it happened.