There could be more then 1 reson for this. According to me the problem might be due to-
More demand due to more user and transaction per minute is much more than previous so there might be less minner as required

After july 2016 halving price of bitcoin had not seen much high on rate but reward had came down to 50% of previous so many minners had sold there system or switched to other coin. So due to less minner and mute user and request per minute the problem exists today.