Why do we follow some Levitical laws and not others regarding things like homosexuality and eating shellfish?

"speak to the sons of Israel..." is a phrase designating instruction only to Israel.  It occurs in many places and is followed by statements and instructions such as...

• Lev. 7:28-29, Then the Lord spoke to Moses, saying, 29 "Speak to the sons of Israel, saying, ‘He who offers the sacrifice of his peace offerings to the Lord shall bring his offering to the Lord from the sacrifice of his peace offerings.'"
• Lev. 12:1-3, Then the Lord spoke to Moses, saying, 2 "Speak to the sons of Israel, saying, ‘When a woman gives birth and bears a male child, then she shall be unclean for seven days, as in the days of her menstruation she shall be unclean. 3 And on the eighth day the flesh of his foreskin shall be circumcised.'"

In many places there is condemnation of the behaviors of the other nations, and their sins are often listed.

• Lev. 18:1-3, Then the Lord spoke to Moses, saying, 2 "Speak to the sons of Israel and say to them, ‘I am the Lord your God. 3 You shall not do what is done in the land of Egypt where you lived, nor are you to do what is done in the land of Canaan where I am bringing you; you shall not walk in their statutes.'"
• Lev. 18:24, "Do not defile yourselves by any of these things; for by all these the nations which I am casting out before you have become defiled."
• Lev. 20:23, "Moreover, you shall not follow the customs of the nation which I shall drive out before you, for they did all these things, and therefore I have abhorred them."

These are things listed for Israel specifically.

1. Exodus 31:13, observe God's sabbaths
2. Lev. 1:2, instruction in bringing an offering to the Lord
3. Lev. 4:2, atonement for unintentional sins
4. Lev. 7:23, don't eat fat from ox, sheep, or goat
5. Lev. 7:29, procedures for peace offering to the Lord
6. Lev. 11:2, list of animals the Israelites may eat
7. Lev. 12:2, uncleanness after giving birth
8. Lev. 15:2, regarding a discharge from a man's body
9. Lev. 23:2,24, rest on 1st day of 7th month
10. Lev. 23:10, bring the harvest first fruits to the priest
11. Lev. 23:34, Feast of Booths on 15th day of 7th month
12. Lev. 24:15, the one cursing God will bear his sin
13. Lev. 25:4, land shall rest from use, 1 every 7 years
14. Lev. 27:2, keep your vows and value
15. Num. 5:6-10, confession of sin and restitution laws
16. Num. 5:11-31, adultery test
17. Num. 6:2-21, Nazirite vows and laws
18. Num. 9:10-14, cleanness and uncleanness regulations
19. Num. 15:2-13, laws for Jews entering Canaan
20. Num. 15:14-31, laws for sojourners
21. Num. 15:38, make tassels for corners of garments
22. Num. 17:1-11, gathering rods from households
23. Num. 19:1-10, ordinance of the Red Heifer
24. Num. 27:8-11, inheritance regulations
25. Num. 33:51-56, instruction upon entering Canaan
26. Num. 35: 9-15, instruction upon entering Canaan
27. Joshua 20:1-6, setting up cities of refuge

These are some of the things the other nations were committing that were forbidden to Israel.  In other words, what the other nations were doing was wrong.

1. Lev. 18:6-18, don't uncover the nakedness of various relatives
2. Lev. 18:19, don't have sexual relations with woman on her period
3. Lev. 18:20; 20:10, don't have intercourse with your neighbor's wife
4. Lev. 18:21; 20:1-5, don't offer children to Molech
5. Lev. 18:22, don't lie with a male as with a female
6. Lev. 18:23 don't have intercourse with animals
7. Lev. 20:6, don't use mediums and spiritists
8. Lev. 20:12, don't lie with daughter in law
9. Lev. 20:13, homosexuality
10. Lev. 20:14, don't marry a woman and her mother
11. Lev. 20:15, don't lie with animals