We have had telescopes since the 1600's why did it take so long to detect pulsars?
In the 1600's Galileo managed to observe the four largest moons
of Jupiter with a telescope.
The smallest of those moons has a diameter of 3,642 kilometers
and a distance of around 600 million kilometers.
By contrast a neutron star/pulsar has a diameter of about 12
kilometers (A small city) and a distance (closest) of 424 light
years (4 x 1015 kilometers).
So they are tiny and a long long way away.