In plain words
For I alone am the enjoyer and Lord of all yajñas. But because they do not know Me in tattva, they fall back.
Word for word
For I [am] the enjoyer and indeed the lord of all yajñas; but they do not know Me in truth; therefore they fall.
The commentators
Bhagavadgītābhāṣyam
कस्मात् ते अविधिपूर्वकं यजन्ते इत्युच्यते ; यस्मात् —
अहं हि सर्वयज्ञानां श्रौतानां स्मार्तानां च सर्वेषां यज्ञानां देवतात्मत्वेन भोक्ता च प्रभुः एव च । मत्स्वामिको हि यज्ञः, ‘अधियज्ञोऽहमेवात्र’(भ. गी. ८ । ४) इति हि उक्तम् । तथा न तु माम् अभिजानन्ति तत्त्वेन यथावत् । अतश्च अविधिपूर्वकम् इष्ट्वा यागफलात् च्यवन्ति प्रच्यवन्ते ते ॥ २४ ॥
In plain words
Why is it said that they worship without the injunction behind them? Because —
I am, in truth, the enjoyer and the lord of every sacrifice there is — whether ordained by śruti or by smṛti, all of them — present as the very self of whatever deity is being invoked. Sacrifice itself belongs to Me; I've already told you as much: 'I alone am the presiding deity of sacrifice here.' And yet people fail to recognize Me as I truly am. That is exactly why, sacrificing without following the proper way, they end up falling short of the very fruit they were reaching for.
Close to the Sanskrit
From what [cause] do they sacrifice not-preceded-by-injunction — thus it is said; because —
For I am indeed the enjoyer and lord of all sacrifices — both those ordained by śruti and those ordained by smṛti, of all sacrifices — by way of being the very self of the deity [invoked]. For sacrifice indeed has Me as its owner; for it has been said: 'I alone am indeed the presiding deity of sacrifice here' (Bhagavad Gītā 8.4). And yet they do not recognize Me properly, in [My] true nature. Therefore, having sacrificed not in accordance with the [proper] injunction, they fall away, [that is,] are deprived, from the fruit of sacrifice.