Santanu did not opt for another marriage because he loved his wife Ganga, so dearly. It was only after her departure, he married Satyavathi. This was what happened.
"Vaisampayana said, 'The maiden then, hearing those soft and sweet
words of the smiling monarch, and remembering her promise to the
Vasus, addressed the king in reply. Of faultless features, the damsel
sending a thrill of pleasure into the heart by every word she uttered,
said, 'O king, I shall become thy wife and obey thy commands. But, O
monarch, thou must not interfere with me in anything I do, be it
agreeable or disagreeable. Nor shall thou ever address me unkindly.
As long as thou shalt behave kindly I promise to live with thee. But
I shall certainly leave thee the moment thou interferest with me or speakest to me an unkind word.'
The king answered, 'Be it so.' And thereupon the damsel obtaining that
excellent monarch, that foremost one of the Bharata race for her
husband, became highly pleased. And king Santanu also, obtaining her
for his wife, enjoyed to the full the pleasure of her company. And
adhering to his promise, he refrained from asking her anything.
And the lord of earth, Santanu, became exceedingly gratified with
her conduct, beauty, magnanimity, and attention to his comforts.
And the goddess Ganga also, of three courses (celestial, terrestrial,
and subterranean) assuming a human form of superior complexion and
endued with celestial beauty, lived happily as the wife of Santanu,
having as the fruit of her virtuous acts, obtained for her husband,
that tiger among kings equal unto Indra himself in splendour.
And she gratified the king by her attractiveness and affection, by her
wiles and love, by her music and dance, and became herself gratified.
And the monarch was so enraptured with his beautiful wife that months,
seasons, and years rolled on without his being conscious of them.
And the king, while thus enjoying himself with his wife, had eight
children born unto him who in beauty were like the very celestials
themselves. But, O Bharata, those children, one after another, as soon
as they were born, were thrown into the river by Ganga who said, 'This
is for thy good.' And the children sank to rise no more.
The king, however, could not be pleased with such conduct. But he
spoke not a word about it lest his wife should leave him.
Please note the following from the above passages.
Santanu, became exceedingly gratified with her conduct, beauty, magnanimity, and attention to his comforts.
But he spoke not a word about it lest his wife should leave him.
The above are self-explanatory.