Perhaps it's not really racism but the point is that Drogba and Lukaku are almost entirely different styles of striker. Lukaku isn't good on the ball and doesn't hold up play well. Lukaku is a pure finisher and that's all he does really well. Drogba on the other hand was very good in his all round game and even if he wasn't scoring he could still be playing very well, the same can't be said about Lukaku. If anything I'd describe Lukaku as being more similar to Chicharito, in the sense that both are exceptional finishers and goalscorers but no other part of their game stands out and neither act that well as the focal point of build up in attack.
So really, it seems like people just compare Lukaku and Drogba because they are big, black and used to play for Chelsea. If Lukaku was white and didn't used to play for Chelsea I'm not sure people would compare the two. Not saying it's necessarily racist but it's clearly not great when people do that