There are probably many answers, but for guesstimating hypothetical missions a look at delta-V/subway maps like this are highly informative in terms of problem scale.
Working from that getting into LEO is 9.4 km/s, these are the rockets you see launching regularly, and can get about 5% of their launch mass into orbit. Earth escape needs another 3.41 km/s, from there getting to a mars intercept is another 0.39 km/s and because mars has an atmosphere we do not need rockets for most of the orbit insert/landing Delta-V, just a reasonably capable heat shield.
If we want a Jupiter intercept it is another 2.7km, and because Europa has very little atmosphere we probably need to use a rocket to descend deep into Jupiter's gravity well rather than aerobraking, meaning for a given booster we get far less mass (possible as little as 5%) to Europa orbit as we can get to Mars for the same mission cost, and need for more complex power, heating and radio systems for a further reduction in 'useful' payload.
The planned Europa Clipper mission uses one of the largest boosters currently available to get 6000kg en route to Jupiter and then burns around 4000kg of fuel getting into orbit (using gravity assists to get more payload than above), with a payload of instruments of 353 kg. Taking this as a template we could redesign the craft to land - using that 353 kg to add landing gear, more fuel and more structure to support the solar panels during landing but suspect we come up with a negative available payload mass. Or we could remove the instruments and make a 300kg lander, which would certainly be possible (similar D/V for a Lunar orbit to Lunar surface) but would be battery powered and very basic.
Getting to Saturn and Enceladus requires even more engine performance, and is beyond the point where solar power is useful adding political costs from launching nuclear RTGs into the mix.
So missions to these places are certainly possible, but the payloads are going to be far smaller than to mars and look far more like Huygens in terms of delivering a couple of pictures and some basic chemistry for a limited period than what we get on mars. Certainly would not be a meaningful search for life, and not much better than the orbiting probes have gathered.
So to date for a given amount of money there has been more return seen taking a complex craft to mars than a more basic craft to the outer moons. As Mars becomes better studied, this is changing hence the progress on Europa Clipper, which at one stage appears to have included a lander at similar cost to an entire mars mission.