John 6:2

Καὶ ἠκολούθει αὐτῷ ὄχλος πολύς, ὅτι ἑώρων αὐτοῦ τὰ σημεῖα ἃ ἐποίει ἐπὶ τῶν ἀσθενούντων.

RBT Greek Interlinear:

Strongs 190; ēkolouthei; ἠκολούθει; were following; V-IIA-3S

Strongs 1161; de; δὲ; and; Conj

Strongs 846; autō; αὐτῷ; to himself; PPro-DM3S

Strongs 3793; ochlos; ὄχλος; Multitude; N-NMS

Strongs 4183; polys; πολύς; great; Adj-NMS

Strongs 3754; hoti; ὅτι; that; Conj

Strongs 2334; etheōroun; ἐθεώρουν*; they were looking attentively; V-IIA-3P

Strongs 3588; ta; τὰ; the; Art-ANP

Strongs 4592; sēmeia; σημεῖα; signs; N-ANP

Strongs 3739; ha; ἃ; ones which; RelPro-ANP

Strongs 4160; epoiei; ἐποίει; was performing; V-IIA-3S

Strongs 1909; epi; ἐπὶ; upon; Prep

Strongs 3588; tōn; τῶν; the; Art-GMP

Strongs 770; asthenountōn; ἀσθενούντων; those who are being weak; V-PPA-GMP

RBT Translation:

Now a crowd multitudinous was accompanying himself because they were gazing attentively at the Signs, whatever he was making, on account of^73e those who are weak.

LITV Translation:
And a great crowd followed Him, for they saw His miracles which He did on the sick ones.

ESV Translation:
And a large crowd was following him, because they saw the signs that he was doing on the sick.

Footnotes

73e

Greek  ἐπὶ τῶν ἀσθενούντων. On the basis of those who are weak.

He is not performing miracles onto sick people, but making signs on account of sick people. τῶν ἀσθενούντων is in the genitive. The genitive of "upon" can be translated "on the basis of," "upon the grounds of," etc, depending on a variety of factors, especially the verb, and of course having the right context. ἐπὶ with a genitive is typically used of location or place, and various relations such as "dependence upon."

See Liddell-Scott for full lexigraphical definitions and usage on ἐπὶ