Bg 4.12

kāṅkṣantaḥ karmaṇāṁ siddhiṁ
yajanta iha devatāḥ
kṣipraṁ hi mānuṣe loke
siddhir bhavati karma-jā

Synonyms: 

kāṅkṣantaḥ — desiring; karmaṇām — of fruitive activities; siddhim — perfection; yajante — they worship by sacrifices; iha — in the material world; devatāḥ — the demigods; kṣipram — very quickly; hi — certainly; mānuṣe— in human society; loke — within this world; siddhiḥ — success; bhavati — comes; karma–jā — from fruitive work.

Translation: 

Men in this world desire success in fruitive activities, and therefore they worship the demigods. Quickly, of course, men get results from fruitive work in this world.

BG 4.12

Here Krishna describes if ultimately He is, who rewards people then why not people worship Him directly.

Why do people attracted to Demigods because they will get quick results. Demigod worship is like contract that has to be obliged. Like child wants chocolate if he is having money he can get it from shopkeeper but mother knows that chocolate will spoil child’s teeth so she will give some other sweet which is good for child to give up slowly so that his teeth is not damaged. {Another example often quoted in preaching is, if a boy is having 50 Rs note and he wanted cigarette he goes to a shopkeeper, the shopkeeper will take money and give him cigarette but he, the same boy goes to his father for this, what father will he do, he will take money and slap him so that next time he will not want cigarette}

Here devatas also refers to some boot licking of powerful person in this material world.

Quick result can go away quickly, and at the end of life everything they have, they have to leave it in this world only.