Why the people don't accept the challenges?

Question

Do they fear from me? In C++, C# etc.

Answer 1

With all due appreciation for facts, fear is a poor choice of word to rationalize with the observation. People may be in a situation or a scenario which simply isn't convenient enough to allow them to accept a challenge - If such situations are predicted to span over 24 hours, a better action of declining a challenge would fit, in order to prevent it from expiring naturally.

Needless to day, it is important for users to possess the freedom to pick and accept/decline their challenges as the see fit.

Answer 2

Still to unsure, no time, don't like challenges in general, not fast enough on mobile (time pressure​) ...
Could have many reasons.